Use VdsmMBackend for lockspace initialization

also move the validation of the host id at validation
stage in order to allow to gather answer file from first host.

Related-To: https://bugzilla.redhat.com/1094657
Change-Id: If439089f4f8efeb02416e041135563415f46a502
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>
Signed-off-by: Jiri Moskovcak <moskovcak@gmail.com>