packaging: package ceph-disk(8)

The ceph-disk man page was added in
a450cab2b8148cb8a9b043d629feccf89e5aabac, but this was not added to the
RPM or DEB packaging. Add it here.

Signed-off-by: Ken Dreyer <kdreyer@redhat.com>
Merge pull request #3061 from ktdreyer/rpm-ceph-disk-man-page

packaging: package ceph-disk(8)

Reviewed-by: Loic Dachary <ldachary@redhat.com>