deb:  Add ceph-coverage to ceph-test deb package

Teuthology uses the ceph-coverage script extensively
and expects it to be installed by the ceph task.  Add
the script to the ceph-test debian package so that it
gets installed for that use case.

Signed-off-by: Sam Lang <sam.lang@inktank.com>
(cherry picked from commit 376cca2d4d4f548ce6b00b4fc2928d2e6d41038f)