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).
@jzoldak and I were discussing this morning. And agree...for now this is ok since we'll only pull from master. But once we need/want to grab other branches, this will need some changes.

JZ the depth setting is what's making it a shallow clone...we only ask for minimal ref-log data.