Add support for package caching via a cache proxy

Having started to use pasha and rucksack more lately I am finding it a bit of a pain to wait for the same packages to be downloaded again and again. This is usually happening as I am trying to finalize my image definition, but could easily be the same scenario when I am building several different images that share many of the same packages. It would be more efficient to cache packages and reuse them as much as possible.

Looking at the mmdebstrap man page they cover several methods to enable a package cache. Here I am enabling the use of a caching proxy server (apt-cacher-ng for my testing) as per one of these recommendations.

The intention is to have rucksack pass the value of the cache_proxy via a command line option and not have it included in the configuration file as using a caching proxy vs. not using a caching proxy has no bearing on the image being produced, it is just a runtime option to make builds more efficient. I will be submitting the related changes for rucksack shortly such that and end-to-end use of the new cache_proxy option can be observed.