The variables placement_developer_mode and placement_venv_download
no longer carry any meaning. This review changes placement 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.
This patch adds the support for the placement service with
a simple functional test which hits the API (which in turn
hits the database and keystone for authorization).