contint: move package_builder setup to its own class
Factor out Debian package building related stuff to a standalone class
contint::package_builder out of contint:📦:labs.
Move the dependency from role class to it, that solves a dependency
definition we had which was set to Class['contint::packages::labs'] when
we really want /var/cache/pbuilder to be a symlink to the mounted
Package building is only done on Jessie instances. Add a requires_os()
to prevent the class from being applied on other distributions.