Issue title: Kubernetes Auth enabled via ENV method always give 403 - permission denied
 Issue body: I am trying to generate pki cert using kubernetes service account.  I have set following ENV variables
VAULT_AUTH_METHOD=kubernetes
VAULT_SIDEKICK_ROLE=example
`/vault-sidekick -cn=pki:pki/issue/example-dot-com:common_name=common.mywebsite.com`
This always gives 
Code: 403. Errors: * 1 error occurred: * permission denied
How ever when tried with "token" based authentication, it was successful.
I have verified kubernetes auth independently using JWT and `v1/auth/kubernetes/login` using curl, it was success. So can you please provide more info on this.

 Comments: 
 Comment 0: Need appropriate policy for the tokens to access the vault path that is used.
Closing this as this is not an issue, how ever proper documentation would be nice.
{'source': 'GithubIssues', 'repo_name_with_owner': 'UKHomeOffice/vault-sidekick', 'issue_id': 532589162, 'repo_stars': 197.0}