Add PostgreSQL pod namespace Scalyr sidecar environment (#196)

Another tiny bit of information that could be useful for log filters
once we start deploying clusters into separate namespaces.

pkg/cluster/k8sres.go

@@ -454,6 +454,15 @@ func (c *Cluster) generatePodTemplate(
 							},
 						},
 					},
+					{
+						Name: "POD_NAMESPACE",
+						ValueFrom: &v1.EnvVarSource{
+							FieldRef: &v1.ObjectFieldSelector{
+								APIVersion: "v1",
+								FieldPath:  "metadata.namespace",
+							},
+						},
+					},
 					{
 						Name:  "SCALYR_API_KEY",
 						Value: c.OpConfig.ScalyrAPIKey,