Clytwynec/removing rake install on jenkins

@jzoldak @benpatterson  Please review...

Is there anyone else that should review this? 

I've tested that devstack is fine with this (but there weren't really any changes to the devstack config).  How would I go about checking this in Jenkins?


This goes with https://github.com/edx/edx-platform/pull/4634
Where does the 'system' var get set?