ceph_test_stress_watch: test over cache pool

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1673 from ceph/wip-stress-watch

ceph_test_stress_watch: test over cache pool

Reviewed-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>