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/compute/servers/test_create_server.py b/tempest/api/compute/servers/test_create_server.py
index e62a52b..c6fb2fb 100644
--- a/tempest/api/compute/servers/test_create_server.py
+++ b/tempest/api/compute/servers/test_create_server.py
@@ -21,6 +21,7 @@
 from tempest.api.compute import base
 from tempest.common.utils import data_utils
 from tempest.common.utils.linux import remote_client
+from tempest.common import waiters
 from tempest import config
 from tempest import test
 
@@ -178,7 +179,8 @@
         # we're OK.
         def cleanup_server():
             self.client.delete_server(server_multi_nics['id'])
-            self.client.wait_for_server_termination(server_multi_nics['id'])
+            waiters.wait_for_server_termination(self.client,
+                                                server_multi_nics['id'])
 
         self.addCleanup(cleanup_server)
 
@@ -218,7 +220,8 @@
 
         def cleanup_server():
             self.client.delete_server(server_multi_nics['id'])
-            self.client.wait_for_server_termination(server_multi_nics['id'])
+            waiters.wait_for_server_termination(self.client,
+                                                server_multi_nics['id'])
 
         self.addCleanup(cleanup_server)