This patch switches the debian/ubuntu OS to build their lxc base
images locally using debootstrap rather than download a pre-built
lxc image. This unifies the approach with Centos-8 which is already
building a local image using dnf.
The LXC cache prestage tasks are removed, and all variables
associated with the download of the lxc image are removed from
defaults/main.yml.
A new variable lxc_apt_mirror is introduced, which is passed to
debootstrap to provide the apt source that the container rootfs
should be built from.
Depends-On: https://review.opendev.org/786396
Change-Id: Ia5a62cee7ab493857df16f7ae906796d22ba616c
786c4fcf18