Issue warning when extra credentials are supplied

It's confusing & misleading when a CLI command such as the following appears to work (this was pulled from IRC):

$ keystone --os-username=admin --token=hastexo --os-auth-url=http://localhost:35357/v2.0 --endpoint=http://localhost:35357/v2.0 user-list
+----------------------------------+--------+---------+-------+
| id | name | enabled | email |
+----------------------------------+--------+---------+-------+
| 336c2f475ce34cbf8f9b2c5438fc76f5 | ec2 | True | |
| 78f4c366872e45ed9ddd

The client should simply issue a warning that the token & endpoint are being used, and that --os-username and --os-auth-url are being ignored.