Add a linter check for environment selectors.

[bergmannf]

This should prevent putting an invalid environment selector like 'stage' or 'int' into this repo.

What type of PR is this?

feature

What this PR does / why we need it?

Additional checks when using environment selectors.

Which Jira/Github issue(s) this PR fixes?

Special notes for your reviewer:

Pre-checks (if applicable):

• Tested latest changes against a cluster

• Included documentation changes with PR

• If this is a new object that is not intended for the FedRAMP environment (if unsure, please reach out to team FedRAMP), please exclude it with:

  matchExpressions:
  - key: api.openshift.com/fedramp
    operator: NotIn
    values: ["true"]

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bergmannf

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[openshift-ci]

@bergmannf: all tests passed!

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