test: catch a straggler still using 'data' pool

Used rbd pool instead, which is still created by default.

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #2142 from ceph/wip-data-pool

test: catch a straggler still using 'data' pool

Reviewed-by: Sage Weil <sage@redhat.com>