Unfortunately the change in [1] broke the integrated build.
The py_pkgs lookup is incapable of interpreting jinja. It
also assumes that any variable named _pip_packages has
python packages in it, and processes them.
This renames the variable so that it conforms with other distro
package lists using _distro_packages instead.
It also adjusts the way the packages are installed a little so
that the var name is overridable, and the variables are nearer
their physical host siblings.
[1] https://review.openstack.org/563684
Change-Id: I2acdab13ba2afe18656cdc66037af6b731058836
d99cf0f0b1