Migrate the service_clients module to tempest.lib.services.clients.
Migrate related unit tests as well.
The clients module atm imports plugin.py from Tempest which is not
allowed via hacking to avoid cirtular dependencies.
If there is no way around this, I will have to remove the self
registration of the service clients from plugins, and ask the
plugins to do the registration themselves - which is a pity. Ideas?
Change-Id: I40e3478f69af62a7cdc14fa65ed21dcfbbe10e72
e07579c603