Shared storage instance failover

Modify LUFailoverInstance to enable shared storage instances to failover.
Shared storage instance failover requires either a target node or an
iallocator to determine the target node. If none is given, the cluster default
iallocator is used.

The hook environment variables {OLD,NEW}_SECONDARY will be blank for shared
storage instances.

Locking behaviour is the same as for instance migration.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
[iustin@google.com: revert the DTS_NET_MIRROR specific changes]
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>