Wire up diskmanager to machine agent

All machine agents now run a diskmanager
worker. There are a few small changes to
the diskmanager itself, too:
 - block devices are now sorted by name
   before being sent to the API server
 - if block devices haven't changed since
   the last time they were sent, they
   will not be sent again
 - block devices that do not exist in
   /dev, but are listed by lsblk, are
   now ignored (this happens in LXC)