Remove multipath mapping device descriptor

When detaching a volume from a instance, we need to remove
multipath mapping device descriptor if there are other volumes
mapping to the host instead of do nothing, for some cinder drivers
can not terminate connection for multipath tool sending IOs to
storage array periodically.

Closes-bug: #1223975
Change-Id: I6fc02e045146ece49162ca6be1a83fa8d0214a0a
Merge "Remove multipath mapping device descriptor"