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
21 files changed