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.

pkg/util/config/config.go

@@ -38,6 +38,7 @@ type Resources struct {
 	NodeReadinessLabel      map[string]string `name:"node_readiness_label" default:""`
 	MaxInstances            int32             `name:"max_instances" default:"-1"`
 	MinInstances            int32             `name:"min_instances" default:"-1"`
+	ShmVolume               bool              `name:"enable_shm_volume" default:"true"`
 }
 
 // Auth describes authentication specific configuration parameters