Update role for new source build process
The variables zun_developer_mode and zun_venv_download
no longer carry any meaning. This review changes zun 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 installation 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.
We also change include_tasks to import_tasks and include_role
to import_role so that the tags in the python_venv_build role
As part of commit xenial testing was removed. Instead centos 7
and opensuse 15 functional tests were added.