vagrant: Add suport for HTTP proxy build settings
We support three options: external proxy, a proxy inside the virtual machine and no proxy at all. The first and the last are both trivial to implement. For the in-VM proxy, we install and configure apt-cacher-ng during provisioning. We do that all the time to support different proxy options once the VM is started. The custom apt-cacher-ng configuration is required to support downloads of `.o` and `.tar.bz2` files by the firmware installers.