Keystone v3 API is CURRENT and the v2 API is deprecated now.
So we need to change the default config of auth_version to fit for
current API status.
NOTE: Devstack changes this config to "v2" when enabling the v2 API.
So we will still test the v2 API on the gate until we will
change Devstack (I801e6740258ddea2a1b628a209970e0307d39d12)
Change-Id: Id5e5ed9bf4f8b0f9eb376bfc7c5801f0956da1d9
a6065a6be6