tools/ceph-detect-init: support RHEL as a platform

tools/ceph-detect-init: support RHEL as a platform

12d94cae2a06fd2ae39379e233588be99bff91de introduced a regression in that it
started returning '' as the codename for most supported distros, and also
(apparently) changed the distro string returned on Red Hat Enterprise Linux (to
"rhel"). As a result, ceph-detect-init began throwing an UnsupportedPlatform
exception on RHEL.

Can not be cherry-picked because ceph-disk and ceph-detect-init are no longer
present in master.

Fixes: 12d94cae2a06fd2ae39379e233588be99bff91de
Fixes: http://tracker.ceph.com/issues/18163
Signed-off-by: Nathan Cutler <ncutler@suse.com>