The variables octavia_developer_mode and octavia_venv_download
no longer carry any meaning. This review changes octavia 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.
Change-Id: I8ec47384778732a1e40392c843d02ea990b85c27
e62b66dcf8