ceph.spec.in: test %preun argument is zero for removal-only operations

ceph.spec.in: test %preun argument is zero for removal-only operations

The %preun section now contains logic for disabling and stopping all the
Ceph systemd units when the ceph package is removed. However, there is no
conditional around it, so the units are disabled and stopped on RPM upgrade
as well as removal.

http://tracker.ceph.com/issues/12685 Fixes: #12685

Signed-off-by: Nathan Cutler <ncutler@suse.com>
Signed-off-by: Tim Serong <tserong@suse.com>