test: generalise default_pools in test_rados

Instead of fragile assumptions about what pools
do or do not exist by default, simply use what
exists at test setup time as a baseline.

Fixes: #8751

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #2078 from ceph/wip-8751

test: generalise default_pools in test_rados

Reviewed-by: Sage Weil <sage@redhat.com>