Add CONF.compute_feature_enabled.volume_backed_live_migration

The test_volume_backed_live_migration test has been skipped for some
time because of a bug [1] which could possibly be resolved with newer
libvirt and qemu package versions available in the pike UCA.

This changes the unconditional skip to a config option controlled skip
so that we can try re-enabling this test on the master branch for nova
changes.

[1] https://bugs.launchpad.net/nova/+bug/1524898

Change-Id: I78ceb1aaa3e96f6b76fa94aba50527022c3e2d45
3 files changed