The variables rally_developer_mode and rally_venv_download
no longer carry any meaning. This review changes rally to
do the equivalent of what developer_mode was all the time,
meaning that it always builds the venv and never requires
the repo server, but it will use a repo server when available.
As part of this, we move the source build out of its own file
because it's now a single task to include the venv build role.
This is just to make it easier to follow the code.
Rally doesn't uses upper-constraints. We rely on stable releases,
for that reason, we don't need vars like rally_git_contraints,
rally_git_repo and rally_git_install_branch.
Change-Id: Iad66086bde1680e4a101f75a17700ab83c459124
3407eb3685