Remove usage of wheels during jenkins worker provisioning.

@jzoldak @clytwynec 

Since the wheels were only being used during the initial buildout of the AMI, I'm setting that aside and re-using  the edx-venv directory if it exists (or create it if it does not exist).
Note that if you want the version to ever be anything other than master, the shallow-clone will break.  Since master is hardcoded for now, this will work.