Support Environment Variables values via secrets

Summary

We need to be able to passing in env variables values using secrets. see below for exmaple

  - name: MM_SQLSETTINGS_DATASOURCEREPLICAS
    valueFrom:
      secretKeyRef:
        key: READER_DB_CONNECTION_STRING
        name: db-credentials

We currently use this variable in PartyBus Mattermost instantiation and we need Bigbang to support it for migration purposes.

I Have open a PR to create support for this specific variable.

/cc @micah.nagel @branden.cobb