prefer identity API v3 over v2 in auth_token
prefer identity API v3 over v2 in auth_token

This reverses the default order of preference between v2 and v3 to use
v3 by default if both are available.

Change-Id: I1a0a3b6c0652e7a762b51538c522e4cb1615bd39
Closes-Bug: 1336056

varunarya10/keystonemiddleware