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).
@benpatterson oh sorry, I should have RTFM'd!
Though @feanil has a good point. It would be worth capturing that info (works because we're hard coding master) in a comment.