Always supply a username to auth_token tests setup

Whilst the v2.Password plugin (which is used by default) will accept a
None value for username this is incorrect behaviour. We should always
provide at least a username so that the plugin can do sensible error
handling if it is not present.

Change-Id: Id790b52f3b06240923f605754c7ffcf4ff136062
Closes-Bug: #1361444
Merge "Always supply a username to auth_token tests setup"