Fix live-migration failure in FC multipath case

Currently, /dev/dm-<NUM> instead of /dev/mapper/<multipath_id> is
used to access multipath FC volumes by Compute Node and
multipath_id in connection_info is not maintained properly and
may be lost during connection refreshing.

This implementation will make source Compute Node and destination
Compute Node fail to disconnect/connect to volumes properly and
result in live-migration failure.

To fix it, /dev/mapper<multipath_id> will be used instead of
/dev/dm-<NUM> to access multipath devices, just like iSCSI multipath
implementation, and logic to preserve the unique (across Compute
Nodes) multipath_id is also added.

Closes-Bug: #1327497
(cherry picked from commit 3ea14e8a70a946dbb162ecafa848e4f2fa29772a)

Conflicts:
	nova/storage/linuxscsi.py
	nova/tests/virt/libvirt/test_libvirt_volume.py
	nova/virt/block_device.py
	nova/virt/libvirt/volume.py

This backport commit adjust oslo.i18n usage to oslo-incubator common code,
due to we didn't have oslo.i18n in icehouse.
And remove unused  variable value dev_str in test_libvirt_volume.py,
it should be deleted but not worth a specific commit in stable/icehouse.

Change-Id: I17f15852c098af88afd270084c62eb87693c60d4
Merge "Fix live-migration failure in FC multipath case" into stable/icehouse