fix: format azure compute gallery image ID

[supershal]

What type of PR is this?

/kind bug
What this PR does / why we need it:
Invalid formatting of compute gallery image id results in Azure machine fails to reconcile when compute gallery image is used.

E1020 05:16:10.545726       1 controller.go:326]  "msg"="Reconciler error" "error"="failed to reconcile AzureMachine: failed to reconcile AzureMachine service virtualmachine: failed to create resource e2e-azure-test-3661259/e2e-azure-test-3661259-control-plane-npm7k (service: virtualmachine): compute.VirtualMachinesClient#CreateOrUpdate: Failure sending request: StatusCode=403 -- Original Error: Code=\"LinkedAuthorizationFailed\" Message=\"The client has permission to perform action 'Microsoft.Compute/galleries/images/versions/read' on scope '/subscriptions/*****-****-****-****-*******/resourceGroups/e2e-azure-test-3661259/providers/Microsoft.Compute/virtualMachines/e2e-azure-test-3661259-control-plane-npm7k', however the linked subscription '%!s(*string=0xc0010f00e0)' was not found. \"" "azureMachine"={"name":"e2e-azure-test-3661259-control-plane-npm7k","namespace":"default"} "controller"="azuremachine" "controllerGroup"="infrastructure.cluster.x-k8s.io" "controllerKind"="AzureMachine" "name"="e2e-azure-test-3661259-control-plane-npm7k" "namespace"="default" "reconcileID"="db366495-2601-4701-8b2c-cd2a41bc43e7"

The SubscriptionID and ResrouceGroup fields are string pointers in AzureComputeGalleryImage
When creating the compute gallery image id, they should be dereferenced to string.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #2746

Special notes for your reviewer:

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Added unit test to verify the bug fix.
before bugfix:

go test -timeout 30s -run ^Test_ComputeImageToSDK$ sigs.k8s.io/cluster-api-provider-azure/azure/converters

--- FAIL: Test_ComputeImageToSDK (0.00s)
    --- FAIL: Test_ComputeImageToSDK/Should_returned_parsed_compute_gallery_image_id (0.00s)
        /gitrepos/cluster-api-provider-azure/azure/converters/image_test.go:169: 
            Expected
                <*compute.ImageReference | 0xc00037f140>: {
                    Publisher: nil,
                    Offer: nil,
                    Sku: nil,
                    Version: nil,
                    ExactVersion: nil,
                    SharedGalleryImageID: nil,
                    CommunityGalleryImageID: nil,
                    ID: "/subscriptions/%!s(*string=0xc000428960)/resourceGroups/%!s(*string=0xc000428950)/providers/Microsoft.Compute/galleries/my-gallery/images/my-image/versions/my-version",
                }
            to equal
                <*compute.ImageReference | 0xc00037f540>: {
                    Publisher: nil,
                    Offer: nil,
                    Sku: nil,
                    Version: nil,
                    ExactVersion: nil,
                    SharedGalleryImageID: nil,
                    CommunityGalleryImageID: nil,
                    ID: "/subscriptions/my-subscription-id/resourceGroups/my-resourcegroup/providers/Microsoft.Compute/galleries/my-gallery/images/my-image/versions/my-version",
                }
FAIL
FAIL	sigs.k8s.io/cluster-api-provider-azure/azure/converters	0.701s
FAIL

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

fix formatting of compute gallery image id

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: supershal / name: Shalin Patel (19c9b68, 5400b25)

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[k8s-ci-robot]

Hi @supershal. Thanks for your PR.

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[mboersma]

/lgtm

Thank you for finding and fixing this @supershal!

[mboersma]

/ok-to-test

[dlipovetsky]

Thanks for adding the unit test @supershal!

[supershal]

/retest

[mboersma]

Looks good, just had a nitpick about the error message string.

[mboersma]

/lgtm
/label tide/merge-method-squash

[mboersma]

/assign @CecileRobertMichon

[CecileRobertMichon]

/lgtm
/approve
/cherry-pick release-1.4
/cherry-pick release-1.5

[k8s-infra-cherrypick-robot]

@CecileRobertMichon: once the present PR merges, I will cherry-pick it on top of release-1.4 in a new PR and assign it to you.

In response to this:

/lgtm
/approve
/cherry-pick release-1.4
/cherry-pick release-1.5

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CecileRobertMichon

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[supershal]

/retest

[supershal]

/retest

[supershal]

/retest

[CecileRobertMichon]

/cherry-pick release-1.5

[k8s-infra-cherrypick-robot]

@CecileRobertMichon: once the present PR merges, I will cherry-pick it on top of release-1.5 in a new PR and assign it to you.

In response to this:

/cherry-pick release-1.5

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[k8s-infra-cherrypick-robot]

@CecileRobertMichon: new pull request created: #2793

In response to this:

/lgtm
/approve
/cherry-pick release-1.4
/cherry-pick release-1.5

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[k8s-infra-cherrypick-robot]

@CecileRobertMichon: new pull request created: #2794

In response to this:

/cherry-pick release-1.5

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