Iniset keystone auth version

Introduces support for suth_version config flag in lib/tempest.
The variable is named TEMPEST_AUTH_VERSION, and it can be set
via localrc, so that the devstack-vm-gate-wrap may control it.

The aim is to setup a keystone v3 based experimental check
job in tempest experimental pipeline.

Partially implements bp multi-keystone-api-version-tests

Change-Id: Ia6832d87308c6c7109e6ae0dbd8dff61134718ee
Merge "Iniset keystone auth version"