Update jenkins-job-builder files for its new upstream release 0.8.0

Release 0.8.0 of jenkins-job-builder includes our patch
for proper matrix job support with architecture and distribution
handling as we need it (see https://review.openstack.org/#/c/73990/).

While at it get rid of our custom Debian packages
(jenkins-job-builder + its python-jenkins dependency).
Instead of using the Debian package use jenkins-job-builder via
virtualenv/pip - at least until it is available in version
>=0.8.0 in Debian/stable (backporting it to Debian/wheezy is just
horrible).