upstart: fix ceph-crush-location default

Behave if it is not specified on ceph.conf.  (init-ceph also falls back
to the default.)

Fixes: #6698
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #803 from ceph/wip-6698

upstart: fix ceph-crush-location default

Reviewed-by: Sage Weil <sage@inktank.com>