Replace update.sh functionality

Currently we internally use a couple shell scripts to parse arguments and make sure a copy of ```ceph-qa-suite``` are checkout out before we run ```teuthology-suite```. That always seemed brittle and, more importantly, not fair to the community since those tools were kept out-of-tree. This PR adds the same functionality to teuthology itself.
why an assert here instead of just removing the destination path?

Also, `/tmp/` is not a good candidate for this. Using Python's `tmpdir` is more robust and will be removed
when you are done (getting rid of the whole problem of cleaning up)

https://docs.python.org/2/library/tempfile.html#tempfile.tempdir