The <id> portion of a --block-device-mapping value can be
the resource name or id since the code uses the appropriate
type-specific find_resource utility to lookup the resource
based on the value given.
This change simply fixes the description of <id> to mention
it's name or id rather than just "UUID". My guess is the
description was originally copied from novaclient where id
must be an id since name resolution does not happen in novaclient.
Change-Id: I567f6f6efb3a3b6d387133d21aa81354b2d753bc
c28ed25e3a