mesh: Use node uuids as storage directory names

mesh: Use node uuids as storage directory names

Instead of keeping track of unique 16bit node identifiers, reuse their
UUIDs to create both storage directories and dbus objects.

Because of that:
 - UUID is no longer stored in the JSON file, it's inferred from the
   directory name instead
 - Join(), CreateNetwork() and ImportLocalNode() APIs return an error if
   given UUID already registered within the daemon

Change-Id: I86032691fa81003c5503880c4bbf9a76aa7bd26e
Signed-off-by: Anupam Roy <anupam.r@samsung.com>