Support 'imagePullSecrets' for Redis image

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Description

Support 'imagePullSecrets' for Redis image to allow image pull via private repo.

Related Issues

Dockerhub limits image pulls could caused unsuccessful deployment of ambassador-redis when download limits have been reached.

Testing

It is a manual test by confirming the image being deployed is from a private repo.

Checklist

• I made sure to update CHANGELOG.md.

  Remember, the CHANGELOG needs to mention:

  • Any new features
  • Any changes to our included version of Envoy
  • Any non-backward-compatible changes
  • Any deprecations

• This is unlikely to impact how Ambassador performs at scale.

  Remember, things that might have an impact at scale include:

  • Any significant changes in memory use that might require adjusting the memory limits
  • Any significant changes in CPU use that might require adjusting the CPU limits
  • Anything that might change how many replicas users should use
  • Changes that impact data-plane latency/scalability

• My change is adequately tested.

  Remember when considering testing:

  • LEGACY MODE TESTS DO NOT RUN FOR EVERY PR. If your change is affected by legacy mode, you need
    to run legacy-mode tests manually (set AMBASSADOR_LEGACY_MODE=true and run the tests).
    (This will be fixed soon.)
  • Your change needs to be specifically covered by tests.
    • Tests need to cover all the states where your change is relevant: for example, if you add a behavior that can be enabled or disabled, you'll need tests that cover the enabled case and tests that cover the disabled case. It's not sufficient just to test with the behavior enabled.
  • You also need to make sure that the entire area being changed has adequate test coverage.
    • If existing tests don't actually cover the entire area being changed, add tests.
    • This applies even for aspects of the area that you're not changing – check the test coverage, and improve it if needed!
  • We should lean on the bulk of code being covered by unit tests, but...
  • ... an end-to-end test should cover the integration points

• I updated DEVELOPING.md with any any special dev tricks I had to use to work on this code efficiently.

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Hi @acookin @esmet ,

Could you help review this PR and see if I am missing anything? Thanks