Include target node in hooks nodes for migration

Include target node in hooks nodes for migration

In case of DRBD, hooks run on both primary (source) and secondary
(target) nodes. To get the same behavior for DTS_EXT_MIRROR, where we
do not have secondary node, we should explicitly add target node to
hooks nodes during instance migration/failover.

CheckPrereq() of TLMigrateInstance runs before BuildHooksManager(),
thus target_node calculated by Iallocator is available under
self._migrater.target_node. Use this value instead of
self.op.target_node which can be None.

Update NEWS and related doc entries.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>