[CNG] [CI] Push mirror images on tag/default branch

What does this MR do?

• Removes tag_and_push_mirror
• Updates tag_and_push to consider a second argument that will be used as the mirror image name if defined
• Updates GitLab CI to pass in the second argument to tag_and_push so the mirror image is pushed in all conditions (including if on a tag or the default branch)

Related issues

Closes gitlab-org/charts/gitlab#2837 (closed)

Related to gitlab-org/gitlab#337638 (closed)

Testing

Confirming that mirrored image is pushed as expected:

• https://gitlab.com/gitlab-org/build/CNG/-/jobs/1474786799#L906

  $ push_tags $GITLAB_REF_SLUG ${CI_JOB_NAME/toolbox/task-runner}
  The push refers to repository [registry.gitlab.com/gitlab-org/build/cng/gitlab-task-runner-ce]

• https://gitlab.com/gitlab-org/build/CNG/-/jobs/1474786797#L947

  $ push_tags $GITLAB_REF_SLUG${IMAGE_TAG_EXT} ${CI_JOB_NAME/toolbox/task-runner}
  The push refers to repository [registry.gitlab.com/gitlab-org/build/cng/gitlab-task-runner-ee]

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated

Closes #2837