generate_objects: use rewind=True in upload_objects

Use the rewind=True argument when uploading objects to make
realistic.py's use of upload_objects compatible with boto>=2.4.1, which
will try to seek to the end of the fp if rewind is not True.
Merge pull request #5 from ceph/wip-migrate-test1

generate_objects: use rewind=True in upload_objects