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
Maintain a local copy of the base class, taken from Tempest with head of
master at c5f1064759fe6c75a4bc5dc251ed1661845936cb.
Change-Id: Idfa5ebe18c794c51e406156fb120d128478d4f1e
