Change compute volume client to return one value and update tests

This included the volume extensions client and the volume-related methods
in the servers client.

Partially implements: blueprint clients-return-one-value

Change-Id: Ice07e38037665def7a8e2982be85ce4df3726311
diff --git a/tempest/api/compute/volumes/test_attach_volume.py b/tempest/api/compute/volumes/test_attach_volume.py
index 7fef52f..40b93a8 100644
--- a/tempest/api/compute/volumes/test_attach_volume.py
+++ b/tempest/api/compute/volumes/test_attach_volume.py
@@ -68,7 +68,7 @@
                                                    'available')
 
         # Attach the volume to the server
-        _, self.attachment = self.servers_client.attach_volume(
+        self.attachment = self.servers_client.attach_volume(
             self.server['id'],
             self.volume['id'],
             device='/dev/%s' % self.device)
@@ -116,13 +116,13 @@
         # Create Server, Volume and attach that Volume to Server
         self._create_and_attach()
         # List Volume attachment of the server
-        _, body = self.servers_client.list_volume_attachments(
+        body = self.servers_client.list_volume_attachments(
             self.server['id'])
         self.assertEqual(1, len(body))
         self.assertIn(self.attachment, body)
 
         # Get Volume attachment of the server
-        _, body = self.servers_client.get_volume_attachment(
+        body = self.servers_client.get_volume_attachment(
             self.server['id'],
             self.attachment['id'])
         self.assertEqual(self.server['id'], body['serverId'])