The technique where /var/cache/apt is mounted as a shared folder conflicts
with vagrant-cachier's workings. Therefore, ignore vagrant-cachier if the
user selects ./makebuildserver's custom apt cache. The shared folder way
has the advantage for CI builds of storing the cache outside of
VAGRANT_HOME, which is set to be in the git project. That gets wiped by
`git clean -fdx` on each CI build.
Vagrantfile is now committed and not changed between configurations. It is
configed by translating the python config file's dict to a YAML file, which
Vagrantfile now loads and uses. This makes it a lot easier for vagrant
users and python programmers to understand, and hopefully makes it easier
to maintain and test with.
