Use InvalidCredentials exception from correct path

InvalidCredentials is in tempest/lib/exceptions.py, not in
tempest/exceptions.py. This patch is to use it from correct path.

Change-Id: I2319dce4f1b5a18707b1e7b51635f4f9e93494be
diff --git a/tempest/clients.py b/tempest/clients.py
index be6bc02..f358d91 100644
--- a/tempest/clients.py
+++ b/tempest/clients.py
@@ -19,7 +19,6 @@
 
 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.lib.services import clients
@@ -350,7 +349,7 @@
     # kwargs for auth provider match the common ones used by service clients
     default_params = config.service_client_config()
     if credentials is None:
-        raise exceptions.InvalidCredentials(
+        raise lib_exc.InvalidCredentials(
             'Credentials must be specified')
     auth_provider_class, auth_url = get_auth_provider_class(
         credentials)