Ensure that vagrant-aws images come online with properly populated apt-caches

We won't need this on production machines since they should be updating their
internal apt-caches on a daily basis, see
<https://issues.jenkins-ci.org/browse/INFRA-31>

[FIXED INFRA-33]
Clean up the profile::buildslave and incorporate it in the celery role

This also adds some relatively basic serverspec testing of what a buildslave
should look like on a real machine

[FIXED INFRA-23]