core: SnapshotStatus - remove BROKEN status

Remove BROKEN SnapshotStatus as it is not used anymore:
* Remove from SnapshotStatus enum.
* Remove SnapshotsValidator -> snapshotNotBroken validation.
* Remove ACTION_TYPE_FAILED_VM_SNAPSHOT_IS_BROKEN VdcBllMessage.

Change-Id: I509e278d2d37fb32a7737bf21111f1bfc7b5019b
Bug-Url: https://bugzilla.redhat.com/1056935
Signed-off-by: Daniel Erez <derez@redhat.com>