Added help note in attach volume dialog

Added help note in Horizon's dashboard (Edit attachments dialog), in order to
inform that device name may not match the proposed one.
The intention is to provide more information when device names don't match due
to hypervisor limitations or naming convention.

Change-Id: I39828542047a2f2149815e2beba1099501aa755e
Partial-Bug: #1217874
Merge "Added help note in attach volume dialog"