ceph-disk: add multipath support

ceph-disk: add multipath support

A multipath device is detected because there is a
/sys/dev/block/M:m/dm/uuid file with the mpath- prefix (or part\w+-mpath
prefix).

When ceph-disk prepares data or journal devices on a multipath device,
it sets the partition typecode to MPATH_JOURNAL_UUID, MPATH_OSD_UUID and
MPATH_TOBE_UUID to

 a) help the udev rules distinguish them from other devices in
    devicemapper
 b) allow ceph-disk to fail if an attempt is made to activate a device
    with this type without accessing it via a multipath device

The 95-ceph-osd.rules call ceph-disk activate on partitions of type
MPATH_JOURNAL_UUID, MPATH_OSD_UUID. It relies on ceph-disk to do nothing
if the device is not accessed via multipath.

http://tracker.ceph.com/issues/11881 Fixes: #11881

Signed-off-by: Loic Dachary <ldachary@redhat.com>