Make v3 update_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 update_role of v3 identity_client use **kwargs.
Also update doc string for the same.

Partially implements blueprint consistent-service-method-names

Change-Id: Ic30ce16d5240e597b5abab1590fbfa251321f2b1
diff --git a/tempest/api/identity/admin/v3/test_roles.py b/tempest/api/identity/admin/v3/test_roles.py
index 3be2643..f1edf9b 100644
--- a/tempest/api/identity/admin/v3/test_roles.py
+++ b/tempest/api/identity/admin/v3/test_roles.py
@@ -75,7 +75,8 @@
         self.assertEqual(role['name'], r_name)
 
         new_name = data_utils.rand_name('NewRole')
-        updated_role = self.client.update_role(new_name, role['id'])['role']
+        updated_role = self.client.update_role(role['id'],
+                                               name=new_name)['role']
         self.assertIn('name', updated_role)
         self.assertIn('id', updated_role)
         self.assertIn('links', updated_role)