Refactoring to add templates

[digiserg]

Refactored to use templates instead of the weird format we had before. This make a lot more sense.

apiVersion: digitalis.io/v1beta1
kind: DbSecret
metadata:
  name: db-secret
spec:
  vault:
    mount: cass000
    role: reaonly
  template:
    CASSANDRA_USERNAME: "{{ .username }}"
    CASSANDRA_PASSWORD: "{{ .password }}"