Instead of copying a common set of code between all the roles,
switch to using a common role which checks whether a deploy host
already has the appropriate venv package. If it does not, build
it on the fly and pull it to the deploy host.
Implementing this does away with the requirement to do builds
on the repo container. Once this has been implemented into all
roles then the repo_build role will be retired.
Depends-On: https://review.openstack.org/556840
Change-Id: I57e87406bee5c7d10aa824f18d3142f8f3ac6ab4
975675b659