Stop depending on scenario test base class

The scenario tests base class from Tempest is not a stable interface
and it's going to be refactored on Tempest side, as notified in

http://lists.openstack.org/pipermail/openstack-dev/2017-February/112938.html

Stop using that class since there's nothing in it used by horizon
scenario test.

Change-Id: I45ef6cef47a6a26e43612ffc6cb8851d19372712
1 file changed