Environment Variable Updates

[v0lkan]

Environment Variable Updates

Description

This update moves environment variables strings to symbolic constants to
make the code less error-prone, and easier to refactor.

Changes

• Moved env vars to ./core/constants/constants.go.
• Sorted the variables alphabetically.
• Created constants for the default values of the env vars oo.
• Updated relevant documentation.
• Cross-checked that all env vars in the code also exist in the public documentation.
• Created constants.GetEnv() that ensures that a constant is already defined
  for an env var to be used.
• Created helpers like constants.True(s) and constants.Never(s).
• Renamed certain environment variables to keep them together when sorted alphabetically.
• updated helm values and regenerated manifests.
• added retries to certain make targets (sometimes, some k8s hooks were not ready and it was resulting in ClusterSPIFFEIDs not reconciling properly; retrying the make command after a 5-10 second wait seemed to have fixed that)

Test Policy Compliance

• I have added or updated unit tests for my changes.
• I have included integration tests where applicable.
• All new and existing tests pass successfully.

Code Quality

• I have followed the coding standards for this project.
• I have performed a self-review of my code.
• My code is well-commented, particularly in areas that may be difficult
  to understand.

Documentation

• I have made corresponding changes to the documentation (if applicable).
• I have updated any relevant READMEs or wiki pages.

Checklist

Before you submit this PR, please make sure:

• You have read the contributing guidelines and
  especially the test policy.
• You have thoroughly tested your changes.
• You have followed all the contributing guidelines for this project.
• You understand and agree that your contributions will be publicly available
  under the project's license.

By submitting this pull request, you confirm that my contribution is made under
the terms of the project's license and that you have the authority to grant
these rights.

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[vmwclabot]

@v0lkan, you must sign every commit in this pull request acknowledging our Developer Certificate of Origin before your changes are merged. This can be done by adding Signed-off-by: John Doe <john.doe@email.org> to the last line of each Git commit message. The e-mail address used to sign must match the e-mail address of the Git author. Click here to view the Developer Certificate of Origin agreement.

• Commit 2df103c must be signed by me@volkan.io