[freezer-dr] allow freezer-dr to use live migration in case it's possible to migrate vms from the compute node

Current version of freezer-dr uses evacuation drivers for dealing with instances. For the cases when instances can be live-migrated from the compute node it might be useful to use live migration instead of evacuation for minimizing downtimes of these instances.
This feature might be useful for compute nodes which are not in a good shape, but live migration is possible.

According to http://docs.openstack.org/admin-guide/compute-configuring-migrations.html nova should be configured in appropriate way before starting live migration.

Here are the guide on how to do a migration itself http://docs.openstack.org/admin-guide/compute-live-migration-usage.html