define ownership between operator and clusters via annotation (#802)

* define ownership between operator and postgres clusters
* add documentation
* add unit test

manifests/postgres-operator.yaml

@@ -35,3 +35,6 @@ spec:
         # In order to use the CRD OperatorConfiguration instead, uncomment these lines and comment out the two lines above
         # - name: POSTGRES_OPERATOR_CONFIGURATION_OBJECT
         #  value: postgresql-operator-default-configuration
+        # Define an ID to isolate controllers from each other
+        # - name: CONTROLLER_ID
+        #   value: "second-operator"