WIP Connection pooler support

Add an initial support for a connection pooler. The idea is to make it
generic enough to be able to switch a corresponding docker image to
change from pgbouncer to e.g. odyssey. Operator needs to create a
deployment with pooler and a service for it to access.

manifests/minimal-postgres-manifest.yaml

@@ -17,3 +17,5 @@ spec:
     foo: zalando  # dbname: owner
   postgresql:
     version: "11"
+  connectionPool:
+    type: "pgbouncer"

manifests/operator-service-account-rbac.yaml

@@ -119,6 +119,7 @@ rules:
   - apps
   resources:
   - statefulsets
+  - deployments
   verbs:
   - create
   - delete