contint: make Jessie slaves package builders
The ::package_builder class setup cowbuilder images suitable for
building packages on all Linux distributions we care about, specially
with customization to grab packages from apt.wikimedia.org
Thus, we can build packages for our Precise flavor on a Jessie host by
simply pointing to the appropriate cow image.
On Jessie hosts, apply ::package_builder.
Require contint::packages::labs to have /var/cache/pbuilder to be a
symlink to the extended disk space mounted on /mnt.