Make v2,v3 create_role use **kwargs & doc string

As we discussed on
http://lists.openstack.org/pipermail/openstack-dev/2015-July/068864.html
All http POST/PUT methods need to contain **kwargs as their arguments.

This patch makes create_role of v2,v3 identity_client use **kwargs.
Also update doc string for the same.

Partially implements blueprint consistent-service-method-names

Change-Id: I876e9e5b8aea37076b180a7857dd79bb6d7c1f10
diff --git a/tempest/common/cred_client.py b/tempest/common/cred_client.py
index a33732e..5d15988 100644
--- a/tempest/common/cred_client.py
+++ b/tempest/common/cred_client.py
@@ -61,7 +61,7 @@
 
     def create_user_role(self, role_name):
         if not self._check_role_exists(role_name):
-            self.roles_client.create_role(role_name)
+            self.roles_client.create_role(name=role_name)
 
     def assign_user_role(self, user, project, role_name):
         role = self._check_role_exists(role_name)