ceph-disk:  udev wrapper for Centos

On Centos 6.3 and other systems with udev versions prior to 172
the gpt label fields are not available for use in udev rules. The
ceph-disk-udev script is a wrapper that extracts the partition
type guid from the label and calls ceph-disk-activate if it is
a ceph guid type.

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