Fix elasticsearch example volume descriptors

This makes the elasticsearch example work with Kubernetes 1.1.x.

examples/elasticsearch/production_cluster/es-client-rc.yaml

@@ -48,5 +48,4 @@ spec:
           name: storage
       volumes:
       - name: storage
-        source:
+        emptyDir: {}
-          emptyDir: {}

examples/elasticsearch/production_cluster/es-data-rc.yaml

@@ -43,5 +43,4 @@ spec:
           name: storage
       volumes:
       - name: storage
-        source:
+        emptyDir: {}
-          emptyDir: {}

examples/elasticsearch/production_cluster/es-master-rc.yaml

@@ -45,5 +45,4 @@ spec:
           name: storage
       volumes:
       - name: storage
-        source:
+        emptyDir: {}
-          emptyDir: {}