enable cinder v2 api for test_multi_backend

    volume_type_client for v2 is added.

Partially implements: blueprint cinder-v2-api-tests

Change-Id: I0729eaf30946bd291f2fdaee8e99bf125e47be83
Move tempest to oslo-config-generator

Also fixes tempest reported bug with old generator in oslo-incubator,
when identical options are registered in different groups.

Change-Id: I274f48503c1b61ca09cabc8d8c50245ba1059943
Closes-bug: 1282887
Merge "enable cinder v2 api for test_multi_backend"
Merge "Move tempest to oslo-config-generator"
Merge branch 'master' of https://github.com/openstack/tempest