Attempt to test live_migration by default

Now that we are starting to try testing multi node devstack we want to
test live_migration. In single node setups the live_migration tests will
be skipped  since the tests make sure there are at least two compute nodes first.

Change-Id: I8e8a940a6eb3fa28adafb1035c549eb6d572baac
Merge "Attempt to test live_migration by default"