debian: replace SysV rbdmap with systemd service

debian: replace SysV rbdmap with systemd service

Stop shipping /etc/init.d/rbdmap in the Debian packages. Ship the
rbdmap.service systemd unit file instead.

The corresponding change has already been made for RPMs, in
9224ac2ad25f7d017916f58b642c0ea25305c3e5.

For Upstart-based systems (eg Ubuntu Trusty), the Debian packages
already contain rbdmap.conf.

(This gets us a tiny bit closer to being able to remove the rbdmap SysV
script from our tree entirely.)

Signed-off-by: Ken Dreyer <kdreyer@redhat.com>