ceph.spec.in: use _udevrulesdir to eliminate conditionals

ceph.spec.in: use _udevrulesdir to eliminate conditionals

The conditionals governing where 50-rbd.rules is installed were not doing the
right thing on SUSE distros.

Start using the %_udevrulesdir RPM macro, while taking care that it is defined
and set to the right value. Use it to eliminate some conditionals around other
udev rules files as well.

http://tracker.ceph.com/issues/12166 Fixes: #12166

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