Shared storage node migration

Modify LUNodeMigrate to provide node migration for nodes with instances using
shared storage. gnt-node migrate has to be passed an iallocator for migration
of shared storage instances to be performed. When using a shared storage
backend, all cluster nodes are locked.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>