Align data returned by deployment graphs collection handler

Change-Id: I6680697f6e046ef0aef0ef4a90a13e7905f0526c
Closes-Bug: 1619237

fuelclient/tests/unit/v2/lib/test_deployment_graph.py

@@ -192,7 +192,7 @@ class TestDeploymentGraphFacade(test_api.BaseLibTest):
     def test_graphs_download_cluster(self):
         matcher_get = self.m_request.get(
             '/api/v1/clusters/1/deployment_graphs/custom_graph',
-            json={'tasks': []}
+            json=[{'tasks': []}]
         )
         self.client.download(env_id=1, level='cluster',
                              graph_type='custom_graph')

fuelclient/v1/graph.py

@@ -144,7 +144,7 @@ class GraphClient(base_v1.BaseV1Client):
             'cluster': lambda: self.get_graph_for_model(
                 related_model='clusters',
                 related_model_id=env_id,
-                graph_type=graph_type).get('tasks', []),
+                graph_type=graph_type)[0].get('tasks', []),
 
             'plugins': lambda: self.get_merged_plugins_tasks(
                 env_id=env_id,