Don't setup identity v3 client if v3 is disabled

Some tests report failures about the identity v3 url not being
specified, even if api_v3 is set to false in tempest.conf.

Change-Id: Ifb5c6c8bd154373d33c90236ff37fcc5b8759405
Closes-Bug: #1389742
Merge "Don't setup identity v3 client if v3 is disabled"