ceph-disk: add '[un]suppress-activate <dev>' command

It is often useful to prepare but not activate a device, for example when
preparing a bunch of spare disks.  This marks a device as 'do not
activate' so that it can be prepared without activating.

Fixes: #3255
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #260 from ceph/wip-suppress

Reviewed-by: Gary Lowell <gary.lowell@inktank.com>