Set xml_api_v2 for Compute tests

Nova XML V2  api support is being removed in Kilo, so we need to
enable here to ensure that the API is still tested in this stable release.

Change-Id: I85ca8f5a7140a2637e64e980015ee30b2a23a46d
Merge branch 'stable/icehouse' of https://github.com/openstack-dev/devstack into stable/icehouse