ceph_test_rados_api_tier: retry EBUSY race checks

...or else these will occasionally fail against a thrashing cluster.

Signed-off-by: Sage Weil <sage@inktank.com>
qa/workunits/rest/test.py: rbd pool ruleset is now 0

Signed-off-by: Sage Weil <sage@inktank.com>