Show docker_volume_size in `cluster show`

docker_volume_size is an parameter for creating cluster, it should also
be displayed when showing it

Change-Id: I354043496cc60a99c362f7d8bf36d5ca27ed6d30

magnumclient/osc/v1/clusters.py

@@ -48,6 +48,7 @@ CLUSTER_ATTRIBUTES = [
     'create_timeout',
     'node_count',
     'discovery_url',
+    'docker_volume_size',
     'master_count',
     'container_version',
     'name',

magnumclient/tests/osc/unit/v1/fakes.py

@@ -253,6 +253,7 @@ class FakeCluster(object):
             'create_timeout': 60,
             'node_count': 1,
             'discovery_url': 'https://fake.cluster',
+            'docker_volume_size': 1,
             'master_count': 1,
             'container_version': None,
             'name': 'fake-cluster',