Improve findSubnet logic

[pydctw]

Make findSubnet to return an error when there is no matching subnet with ID

What type of PR is this?
/kind failing-test

What this PR does / why we need it:
Make findSubnet to return an error when there is no matching subnet with ID

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Part of #3399

Special notes for your reviewer:
Wrote a test case that catches the failing scenario described in the issue

Checklist:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

[k8s-ci-robot]

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[Ankitasw]

/test pull-cluster-api-provider-aws-e2e

[Ankitasw]

/lgtm

[pydctw]

e2e test failure - recently merged PR (#3425) is causing a failure. Not related to changes in this PR.

Failed to run clusterctl config cluster
Unexpected error:
    <*errors.withStack | 0xc000fd39e0>: {
        error: <*errors.withMessage | 0xc0015d2880>{
            cause: <*errors.fundamental | 0xc000fd39b0>{
                msg: "a lowercase RFC 1123 subdomain must consist of lower case alphanumeric characters, '-' or '.', and must start and end with an alphanumeric character (e.g. 'example.com', regex used for validation is '[a-z0-9]([-a-z0-9]*[a-z0-9])?(\\.[a-z0-9]([-a-z0-9]*[a-z0-9])?)*')",
             ...
            msg: "invalid cluster name",
        },
        ...
    }
    invalid cluster name: a lowercase RFC 1123 subdomain must consist of lower case alphanumeric characters, '-' or '.', and must start and end with an alphanumeric character (e.g. 'example.com', regex used for validation is '[a-z0-9]([-a-z0-9]*[a-z0-9])?(\.[a-z0-9]([-a-z0-9]*[a-z0-9])?)*')
occurred
/home/prow/go/pkg/mod/sigs.k8s.io/cluster-api/test@v1.1.2/framework/clusterctl/client.go:216}

[k8s-ci-robot]

New changes are detected. LGTM label has been removed.

[pydctw]

Will wait for #3429 to be merged.

[pydctw]

/test pull-cluster-api-provider-aws-e2e

[sedefsavas]

Using same log for having no public subnets in the list vs having public subnets but missing their subnet IDs is misleading. Might worth keeping if len(subnets) == 0 check.

[sedefsavas]

Instead of returning early when the ID is found, checking negative case first and returning early if fails is a more readable pattern to use and by this way calling getSubnetId() twice in the same case statement will not be needed.

[sedefsavas]

Same comment as above.

[sedefsavas]

Same comment as above.

[sedefsavas]

What is the status with this PR?
If this PR is no longer required considering the fix coming up in cluster-api, let's close this one.