base_docker_image: update MongoDB, use Debian sources
base_docker_image: update MongoDB, use Debian sources

_/python:2 inherits from debian:jessie, so we should use the
debian apt sources for mongodb

nextml/NEXT,nextml/NEXT,nextml/NEXT,kgjamieson/NEXT,nextml/NEXT,kgjamieson/NEXT,kgjamieson/NEXT,kgjamieson/NEXT