CLI changes to facilitate shared storage migration/failover

Add DST_NODE_OPT to cli.py to use for directly specifying the target node
during migration/failover.

gnt-instance failover/migrate also get passed an iallocator option.

gnt-node failover/migrate get only a target_node option.

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