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/stress/actions/ssh_floating.py b/tempest/stress/actions/ssh_floating.py
index 09e6d88..2a7a85c 100644
--- a/tempest/stress/actions/ssh_floating.py
+++ b/tempest/stress/actions/ssh_floating.py
@@ -86,7 +86,8 @@
     def _destroy_vm(self):
         self.logger.info("deleting %s" % self.server_id)
         self.manager.servers_client.delete_server(self.server_id)
-        self.manager.servers_client.wait_for_server_termination(self.server_id)
+        waiters.wait_for_server_termination(self.manager.servers_client,
+                                            self.server_id)
         self.logger.info("deleted %s" % self.server_id)
 
     def _create_sec_group(self):