Rewrite roundtrip test based on the readwrite one.
Rewrite roundtrip test based on the readwrite one.

They probably should share more code in the future, but it's only
200 lines of python, so I'm not all that worried.

dreamhost/s3-tests,tobegit3hub/s3-tests,piousbox/s3-tests,Intel-bigdata/s3-tests,SUSE/s3-tests,Nexenta/s3-tests,Nexenta/s3-tests,Nexenta/s3-tests,andrewgaul/s3-tests,ceph/s3-tests,jmunhoz/s3-tests,ceph/s3-tests,Intel-bigdata/s3-tests,SUSE/s3-tests,piousbox/s3-tests,robbat2/s3-tests,tobegit3hub/s3-tests,Nexenta/s3-tests,basho/s3-tests,robbat2/s3-tests,smithfarm/s3-tests,andrewgaul/s3-tests,piousbox/s3-tests,timuralp/s3-tests,dreamhost/s3-tests,jmunhoz/s3-tests,timuralp/s3-tests,basho/s3-tests,smithfarm/s3-tests