Update cosbench.py to add cosbench user/subuser if use_existing is False

[wadeholler]

Update cosbench.py to add cosbench user/subuser if use_existing is False

--There may be more elegant ways to update radosgw with the cosbench:operator sub user but this follows the cosbench documentation.
--Only update is use_existing is False

Issue #64

While we are at it. Add a simple cosbench example per Issue #43

[markhpc]

I'm going to merge this for now, but I think we should try to keep rados stuff out of the benchmarks, so we may want to move this into the ceph cluster class and have the benchmark use some kind of generic interface for creating users and whatnot. The eventual goal would be that we could create a swift backend in addition to ceph and use the same benchmark.