Change data_processing client to return one value and update tests

Two methods returned string data and so were left as returning resp,data

Partially implements: blueprint clients-return-one-value

Change-Id: I08c8e8b3dd61e85afca174a8e4b3148e7ac6452a
diff --git a/tempest/api/data_processing/test_plugins.py b/tempest/api/data_processing/test_plugins.py
index 4b4ec48..8c9b720 100644
--- a/tempest/api/data_processing/test_plugins.py
+++ b/tempest/api/data_processing/test_plugins.py
@@ -25,7 +25,7 @@
 
         It ensures main plugins availability.
         """
-        _, plugins = self.client.list_plugins()
+        plugins = self.client.list_plugins()
         plugins_names = [plugin['name'] for plugin in plugins]
         for enabled_plugin in CONF.data_processing_feature_enabled.plugins:
             self.assertIn(enabled_plugin, plugins_names)
@@ -39,12 +39,12 @@
     @test.attr(type='smoke')
     def test_plugin_get(self):
         for plugin_name in self._list_all_plugin_names():
-            _, plugin = self.client.get_plugin(plugin_name)
+            plugin = self.client.get_plugin(plugin_name)
             self.assertEqual(plugin_name, plugin['name'])
 
             for plugin_version in plugin['versions']:
-                _, detailed_plugin = self.client.get_plugin(plugin_name,
-                                                            plugin_version)
+                detailed_plugin = self.client.get_plugin(plugin_name,
+                                                         plugin_version)
                 self.assertEqual(plugin_name, detailed_plugin['name'])
 
                 # check that required image tags contains name and version