Add a skeletal CLI to help design the high-level operations.

We have 3 groups of commands:
  attach, detach: to take a volume online and offline locally
  use, free: to allow us to share free space with other nodes
  status: to display the (local) status

These commands are all localhost-only. Safe operation relies on
a cluster-level entity ensuring that
  1. volumes are only ever online on one host at a time
  2. free space is never 'used' on more than one host at a time

Signed-off-by: David Scott <dave.scott@eu.citrix.com>