test_rbd.py: ignore children in cache pools

This is necessary until http://tracker.ceph.com/issues/8187 is fixed.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge pull request #1720 from jdurgin/wip-list-children-test

test_rbd.py: ignore children in cache pools

Reviewed-by: Sage Weil <sage@inktank.com>