logrotate.conf: fix osd log rotation under upstart

In commit 7411c3c6a42bef5987bdd76b1812b01686303502 we generalized this
enumeration code by copying what was in the upstart scripts.  However,
while the mon and mds directories get a 'done' file, the OSDs get a 'ready'
file.  Bah!  Trigger off of either one.

Backport: firefly
Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2116 from ceph/wip-logrotate

logrotate.conf: fix osd log rotation

Reviewed-by: Dan Mick <dan.mick@inktank.com>