qa/workunits/rest/test.py: fix rd_kb -> rd_bytes

Signed-off-by: Sage Weil <sage@redhat.com>
qa/workunits/rest/test.py: do snap test on our data2/3 pool

This way it works when a 'data' pool doesn't already exist.

Signed-off-by: Sage Weil <sage@redhat.com>
qa/workunits/rest/test.py: use rbd instead of data pool for size tests

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