logrotate.conf: fix osd log rotation

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>