ceph-disk: add Scientific Linux as a Redhat clone

Scientific Linux is a RHEL clone and needs to use partx.

Signed-off-by: Dan van der Ster <daniel.vanderster@cern.ch>
Merge pull request #2580 from cernceph/wip-scientific

ceph-disk: add Scientific Linux as a Redhat clone

Reviewed-by: Sage Weil <sage@redhat.com>