E2E tests for connection pool

Turn off enableConnectionPool to be able to test it from the scratch.
Add turn on, modify, turn off/on test.

manifests/configmap.yaml

@@ -15,7 +15,7 @@ data:
   # connection_pool_default_cpu_request: "1"
   # connection_pool_default_memory_limit: 100m
   # connection_pool_default_memory_request: "100Mi"
-  # connection_pool_image: ""
+  connection_pool_image: "registry.opensource.zalan.do/acid/pgbouncer:master-3"
   # connection_pool_number_of_instances: 1
   # connection_pool_mode: "transaction"
   # connection_pool_schema: "pooler"

manifests/minimal-postgres-manifest.yaml

@@ -17,4 +17,3 @@ spec:
     foo: zalando  # dbname: owner
   postgresql:
     version: "11"
-  enableConnectionPool: true