Comment 2 for bug 1627640

This was a side effect of the Debian and Ubuntu versions of the ceph package having a ceph-mon systemd unit file; when upstream added systemd support this conflicted.

That said there is probably no reason why the other targets can't be installed anyway; we'll have to look at the ceph-mon change specifically as it will potentially break upgraders who are currently using the ceph-mon unit (rather than target).