PostgreSQL: Allow one to provide her own db configuration secret

If a user provide its own secret by using the
`tower_postgres_configuration_secret` use this variable, else try to use
`<instance>-postgres-configuration` secret. Else create one and use a
local database.

ansible/templates/crd.yml.j2

@@ -26,25 +26,3 @@ spec:
         spec:
           type: object
           properties:
-            external_database:
-              type: boolean
-              description: |
-                If true you must supply a secret containing the location and credentials for
-                connecting to the external database by a user who has permission to create
-                and apply a schema.
-
-                The secret should have the name: <custom resource name>-postgres-configuration and
-                should look like:
-
-                apiVersion: v1
-                kind: Secret
-                metadata:
-                  name: <crname>-postgres-configuration
-                  namespace: <target namespace>
-                stringData:
-                  host: <external ip or url resolvable by the cluster>
-                  port: <external port, this usually defaults to 5432>
-                  database: <desired database name>
-                  username: <username to connect as>
-                  password: <password to connect with>
-                type: Opaque