Adding ability to populate -init-auth-method from a secret

[barrymars]

Changes proposed in this PR: https://github.com/hashicorp/consul-k8s/issues/1129

How I've tested this PR:

How I expect reviewers to test this PR:

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...)