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).
It's staying there right now. I had been thinking we can leave it there in case we do end up needing it to not-be shallow. But, considering my previous comment, perhaps we should just clean this up after it's done. I'll update the PR when I get that done.