Provide compatibility methods with Zemke-Rhyme for Vault

Summary:
Provide get_username, get_token, get_password main method,
and an helper method to get a sentry DSN, fully compatible
with Zemke-Rhyne signatures.

An advantage on ZR is each node has only access to the secrets
relevant for this role, so the OpenSearch server doesn't have
access to Docker containers credentials and vice-versa.

Ref T1425

Test Plan: salt-call credentials.get_secret

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1425

Differential Revision: https://devcentral.nasqueron.org/D2649