Backport of added imagePullPolicy for images in values.yaml into release/1.0.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #2310 to be assessed for backporting due to the inclusion of the label backport/1.0.x.

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The person who merged in the original PR is:
@aahel
This person should manually cherry-pick the original PR into a new backport PR,
and close this one when the manual backport PR is merged in.

merge conflict error: POST https://api.github.com/repos/hashicorp/consul-k8s/merges: 409 Merge conflict []

The below text is copied from the body of the original PR.

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Changes proposed in this PR:

• Added support for a global imagePullPolicy that would be used in all images which are configurable using the helm template.

How I've tested this PR:
rendered helm template and test by passing different values of imagePullPolicy
How I expect reviewers to test this PR:
eg:-
cd charts/consul
helm template
-s templates/api-gateway-controller-deployment.yaml
--set 'apiGateway.enabled=true'
--set 'global.imagePullPolicy="Always"'
--set 'apiGateway.image=bar'
--set 'global.enableConsulNamespaces=true'
. | tee /dev/stderr

Checklist:

• Tests added
• CHANGELOG entry added

  HashiCorp engineers only, community PRs should not add a changelog entry.
  Entries should use present tense (e.g. Add support for...)

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Overview of commits

• 873a317 - 7d233ec - 7f968dc - 50ff429 - 2e0e33c - 5b418e1 - 31e23ad - 473a18b - a4c9d96 - 6e4e1b8 - ca9b271

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• aahel
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