Allow configuration of direct-snapshot feature

Current behaviour of nova for performing snapshot for libvirt is: trying to perform a "direct-snapshot" (with rbd backend it consist on directly create an rbd image in images pool from the vms pool), and if it failed for any reason (not implemented in the driver, image not compatible, permission issue...), it fallback to the standard snapshot process, meaning download the disk in local in the compute node, then call glance api to register a new image from this file.

In some case, by example when migrating from a backend to another, we don't want nova to try and achieve a direct-snapshot workflow, because it does not honor glance backend configuration, and there is no way to force nova to use regular snapshot workflow.

In order to allow that, I propose to add a new boolean configuration option in the [libvirt] section, allow_direct_snapshot, which will be checked before trying a direct snapshot. This configuration option will be defaulted to True to not break the existing behaviour.