ceph-disk: implement 'activate-journal'

ceph-disk: implement 'activate-journal'

Activate an osd via its journal device.  udev populates its symlinks and
triggers events in an order that is not related to whether the device is
an osd data partition or a journal.  That means that triggering
'ceph-disk activate' can happen before the journal (or journal symlink)
is present and then fail.

Similarly, it may be that they are on different disks that are hotplugged
with the journal second.

This can be wired up to the journal partition type to ensure that osds are
started when the journal appears second.

Include the udev rules to trigger this.

Signed-off-by: Sage Weil <sage@inktank.com>