mon/PGMap: make generated test instances obey new invariant

As of 091809b8149c7595cbcca439c5b8b75a0c42efe1 we keep an osd_map epoch
for any osd_stat update, and assert as much.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge git://github.com/ceph/ceph