common: Automatize UPSTREAM_DISTRO name based on image name
Similar to what is already done with DISTRO, which points to given image of ours based on name. This time we do the same with upstream images, such as debian:stretch or centos:centos8.
This way, for instance calling docker_images_require "osmo-bsc-latest-centos8" would try to build the osmo-bsc-latest/Dockerfile file starting from a centos8 image.
