Support Vault Auth method

[Kryvchun]

This PR adds support for vault Kubernetes authentication.

Issue: #274.

New parameters:

  -vault-k8s-auth-role-name
      Enable k8s auth method and use this role name

  -vault-k8s-service-account-token
      Use this service token for the k8s auth method

  -vault-k8s-service-account-token-path
      Use the service token from the file for the k8s auth method

  -vault-k8s-service-mount-path
      Use this login mount path for the k8s auth method

[ghost]

LGTM

[Kryvchun]

@eikenb Hi, it is ready for review.

[eikenb]

With the release of consul-template 0.29.1 that included the changes needed for this and the dependency has already been updated.

IE. I'll be reviewing this soon for inclusion in 0.13.0.

[eikenb]

With the one change (.gitignore) this looks good.

Thanks!

[Kryvchun]

@eikenb
.gitignore fixed and main merged.
Thank you