Create ResourceGroup with volumes only if it is required

If volumes_per_node or volumes_size is 0, there is no sense
in creating empty ResourceGroup.

Also remove volumes from one node group in gate to be sure
that sahara correctly handle such case.

Closes-bug: 1494650
Change-Id: Ic592710773b9338f5857e431b0567bc5c87af2bd

etc/scenario/gate/fake.yaml.mako

@@ -18,8 +18,6 @@ clusters:
         node_processes:
           - datanode
           - jobtracker
-        volumes_per_node: 2
-        volumes_size: 1
         auto_security_group: true
     cluster_template:
       name: fake01

sahara/service/heat/templates.py

@@ -310,6 +310,7 @@ class ClusterStack(object):
             }
         })
 
+        if ng.volumes_per_node > 0 and ng.volumes_size > 0:
             resources.update(self._serialize_volume(ng))
 
         return resources