Support iSCSI live migration for different iSCSI target

Currently, Nova premises a situation which iSCSI target is not changed
before and after live migration. Therefore, if destination node has
different iSCSI target, the live migration fails in current Nova's
specification. However, if each compute node uses same iSCSI target,
each compute node recognizes all volumes in the iSCSI target and this
is undesirable situation from the view point of security.

Therefore, this spec proposes to support live migration of instances with
Cinder volumes among compute nodes that need to log into different
iSCSI targets to access the volumes.

For your reference, if iSCSI storages have features to manage visibility
of LUNs for each initiator within one iSCSI target, using same iSCSI
target for each compute node is not a problem. But general iSCSI storages
don't have such kind features, therefore they need this feature to
support live migration.

Change-Id: I9f0212cb74804e0122200ff543c46f1a91a39921
Blueprint iscsi-live-migration-different-target
Merge "Support iSCSI live migration for different iSCSI target"