mirrors.list and appurtenant cleanup scripts should be saved in the mirror location, not the host filesystem

currently, maniacs-setup writes mirrors.list to /etc:

MIRROR_LIST="/etc/apt/mirror.list"

This should actually be stored in the mirror dir itself and the post-mirror scripts. This way you can create the mirror on an external or removable drive, move it to the maas server, and then move it back and be able to use apt-mirror from ANY system that is internet connected.

Currently, the way it's set up, if I:

Install MAAS using maniacs-setup and create a mirror on a non-connected machine
use USB drive to hold the mirror
unplug the USB drive and take it to a laptop
attempt to update the mirror

I will fail, unless that laptop ALSO has a copy of /etc/apt/mirrors.list and the various post-install stuff.

Moving all this into the mirror location itself means that anywhere I take that disk, I'll be able to run apt-mirror and update my mirror.