StatefulSet fsGroup config option to allow non-root spilo (#531)

* StatefulSet fsGroup config option to allow non-root spilo

* Allow Postgres CRD to overide SpiloFSGroup of the Operator.

* Document FSGroup of a Pod cannot be changed after creation.

manifests/complete-postgres-manifest.yaml

@@ -37,6 +37,7 @@ spec:
     limits:
       cpu: 300m
       memory: 300Mi
+  # spiloFSGroup: 103
   patroni:
     initdb:
       encoding: "UTF8"

manifests/postgresql-operator-default-configuration.yaml

@@ -24,6 +24,7 @@ configuration:
     cluster_domain: cluster.local
     oauth_token_secret_name: postgresql-operator
     pod_role_label: spilo-role
+    # spilo_fsgroup: 103
     spilo_privileged: false
     cluster_labels:
         application: spilo