global: create /var/run/ceph on daemon startup

This handles cases where the daemon is started without the benefit of
sysvinit or upstart (as with teuthology or ceph-fuse).

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 654299108bfb11e7dce45f54946d1505f71d2de8)