Add a new clustered disk

Add a new clustered disk

A clustered disk is added by the traditional --add sequence.
However, other nodes need to acknowledge that they can "see"
the device. This is done by --cluster-confirm:

--cluster-confirm SLOTNUM:/dev/whatever (if disk is found)
or
--cluster-confirm SLOTNUM:missing (if disk is not found)

The node initiating the --add, has the disk state tagged with
MD_DISK_CLUSTER_ADD and the one confirming tag the disk with
MD_DISK_CANDIDATE.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: NeilBrown <neilb@suse.de>