Add response assertions and remove unused variables in volume tests

This patch adds response assertions where the response body is declared
but never used in some volume api tests. And also some unused variables
are removed to make code clean.

Change-Id: I29e6e8f1dac42b33d2ac020f682f57db9218115a
diff --git a/tempest/api/volume/test_volume_transfers.py b/tempest/api/volume/test_volume_transfers.py
index 2c13a3c..4108da5 100644
--- a/tempest/api/volume/test_volume_transfers.py
+++ b/tempest/api/volume/test_volume_transfers.py
@@ -59,6 +59,8 @@
         # Accept a volume transfer by alt_tenant
         body = self.alt_client.accept_volume_transfer(
             transfer_id, auth_key=auth_key)['transfer']
+        for key in ['id', 'name', 'links', 'volume_id']:
+            self.assertIn(key, body)
         waiters.wait_for_volume_resource_status(self.alt_volumes_client,
                                                 volume['id'], 'available')
         accepted_volume = self.alt_volumes_client.show_volume(