As long as nova already supports an Identity v3 auth flow when talking
to ironic (Id837d26bb21c158de0504627e488c0692aef1e24), make it use
v3 by default.
This way we don't fail in a keystone v3-only situation, for
example.
Change-Id: I028dfb52108d0630f47a53f8b420b70d4979eb55