Add shm_volume option (#427)

Add possibility to mount a tmpfs volume to /dev/shm to avoid issues like
[this](https://github.com/docker-library/postgres/issues/416). To achieve that
two new options were introduced:

* `enableShmVolume` to PostgreSQL manifest, to specify whether or not mount
this volume per database cluster

* `enable_shm_volume` to operator configuration, to specify whether or not mount
per operator.

The first one, `enableShmVolume` takes precedence to allow us to be more flexible.

manifests/complete-postgres-manifest.yaml

@@ -13,12 +13,13 @@ spec:
     - superuser
     - createdb
   enableMasterLoadBalancer: true
-  enableReplicaLoadBalancer: true 
+  enableReplicaLoadBalancer: true
   allowedSourceRanges: # load balancers' source ranges for both master and replica services
   - 127.0.0.1/32
   databases:
     foo: zalando
 #Expert section
+  enableShmVolume: true
   postgresql:
     version: "10"
     parameters: