tests: add require_jewel_osds before upgrading last hammer node

tests: add require_jewel_osds before upgrading last hammer node

Note: this commit was inspired by
http://github.com/ceph/ceph-qa-suite/commit/50758a4810794d265c5d36a71d1e16799251a00d

As of 10.2.4, when upgrading a cluster from hammer to jewel, after the last
node is upgraded the MON will put the cluster into HEALTH_WARN and say: "all
OSDs are running jewel or later but the 'require_jewel_osds' osdmap flag is not
set". The release notes say:

    This is a signal for the admin to do "ceph osd set require_jewel_osds" – by
    doing this, the upgrade path is complete and no more pre-Jewel OSDs may be
    added to the cluster.

Fixes: http://tracker.ceph.com/issues/18719
Signed-off-by: Nathan Cutler <ncutler@suse.com>