Move wait_for_server_termination from servers_client

wait_for_server_termination of compute client is just a wrapper.
So the method seems redundant and it doesn't fit as service clients.

This patch moves wait_for_server_termination to waiters module which
contains similar methods.

Partially implements blueprint consistent-service-method-names

Change-Id: Ib738ad91bb5e7344b6a342f23f867df42b833608
diff --git a/tempest/api/volume/test_volumes_actions.py b/tempest/api/volume/test_volumes_actions.py
index 067c0c1..58c5ba9 100644
--- a/tempest/api/volume/test_volumes_actions.py
+++ b/tempest/api/volume/test_volumes_actions.py
@@ -53,7 +53,8 @@
     def resource_cleanup(cls):
         # Delete the test instance
         cls.servers_client.delete_server(cls.server['id'])
-        cls.servers_client.wait_for_server_termination(cls.server['id'])
+        waiters.wait_for_server_termination(cls.servers_client,
+                                            cls.server['id'])
 
         super(VolumesV2ActionsTest, cls).resource_cleanup()