Migrate service_clients to tempest.lib

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
diff --git a/tempest/clients.py b/tempest/clients.py
index 680976d..406f9d5 100644
--- a/tempest/clients.py
+++ b/tempest/clients.py
@@ -14,15 +14,13 @@
 #    under the License.
 
 import copy
-
 from oslo_log import log as logging
-
 from tempest.common import negative_rest_client
 from tempest import config
 from tempest import exceptions
 from tempest.lib import auth
 from tempest.lib import exceptions as lib_exc
-from tempest import service_clients
+from tempest.lib.services import clients
 from tempest.services import baremetal
 from tempest.services import data_processing
 from tempest.services import identity
@@ -34,7 +32,7 @@
 LOG = logging.getLogger(__name__)
 
 
-class Manager(service_clients.ServiceClients):
+class Manager(clients.ServiceClients):
     """Top level manager for OpenStack tempest clients"""
 
     default_params = config.service_client_config()
@@ -109,8 +107,8 @@
         # Setup the parameters for all Tempest services.
         # NOTE(andreaf) Since client.py is an internal module of Tempest,
         # it doesn't have to consider plugin configuration.
-        all_tempest_modules = (set(service_clients.tempest_modules()) |
-                               service_clients._tempest_internal_modules())
+        all_tempest_modules = (set(clients.tempest_modules()) |
+                               clients._tempest_internal_modules())
         for service in all_tempest_modules:
             try:
                 # NOTE(andreaf) Use the unversioned service name to fetch