Update references to auth_token middleware

There's references to the auth_token middleware in keystoncelient.
The auth_token middleware has been moved to keystonemiddleware and
the version in keystoneclient shouldn't be used anymore.

If these references aren't updated, then when options are changed in
keystonemiddleware.auth_token the heat-api will fail to start because
there's duplicate options in keystoneclient.middleware.auth_token.

Change-Id: I04573aa5ff967afe3e00329f797fcc71b779e7b3
Closes-Bug: #1379082