This patch eliminates variables related to developer mode as they
don't mean anything anymore with python_venv_build.
It merges the package that was there (git) with devel distro packages
variable. We also remove it from `neutron_package_list` to make sure
we don't install it all the time.
Change-Id: Ie9e236e56dbdb98aacf16de0741584b1483e23c3