Update role for new source build process
The variables magum_developer_mode and magnum_venv_download
no longer carry any meaning. This review changes magnum 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.