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.

docs/reference/cluster_manifest.md

@@ -58,6 +58,13 @@ These parameters are grouped directly under  the `spec` key in the manifest.
   custom docker image that overrides the **docker_image** operator parameter.
   It should be a [Spilo](https://github.com/zalando/spilo) image.  Optional.
 
+* **spiloFSGroup**
+  the Persistent Volumes for the spilo pods in the StatefulSet will be owned
+  and writable by the group ID specified. This will override the **spilo_fsgroup**
+  operator parameter. This is required to run Spilo as a non-root process, but
+  requires a custom spilo image. Note the FSGroup of a Pod cannot be changed
+  without recreating a new Pod.
+
 * **enableMasterLoadBalancer**
   boolean flag to override the operator defaults (set by the
   `enable_master_load_balancer` parameter) to define whether to enable the load

docs/reference/operator_parameters.md

@@ -228,6 +228,11 @@ configuration they are grouped under the `kubernetes` key.
   that should be assigned to the Postgres pods. The priority class itself must
   be defined in advance. Default is empty (use the default priority class).
 
+* **spilo_fsgroup**
+  the Persistent Volumes for the spilo pods in the StatefulSet will be owned and writable by the group ID specified.
+  This is required to run Spilo as a non-root process, but requires a custom spilo image. Note the FSGroup of a Pod
+  cannot be changed without recreating a new Pod.
+
 * **spilo_privileged**
   whether the Spilo container should run in privileged mode. Privileged mode is
   used for AWS volume resizing and not required if you don't need that