Add a new gnt-node command list-drbd

This uses confd to query the DRBD minors, which is very special; no
other command currently does so.

Since the backend is only implemented in the Haskell version of confd,
we have checks that 1) confd is enable, and 2) hs confd is also
enabled. If by mistake people do manage to query Python confd, the
error message will be clean:

  Query gave non-ok status '2': not implemented

So nothing breaks in an "ugly" way.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>