Add keystone api version detection to verify_tempest_config

This commit adds a check for the keystone api versions that are
supported and checks that with the config file. In the process of
adding the additional check the version verification code is reworked
to use a common get versions method.

Partially implements bp config-verification

Change-Id: Ibb15c53e2b2a1e2a2ad95351692d06389f2ebb06