WIP: .gitlab-ci.yml: Use debian:stretch instead python:2.7-slim by default
python:2.7-slim install python in /usr/local instead /usr, which is causing problems with our extensions that expect python to be in /usr/bin/python
python:2.7-slim install python in /usr/local instead /usr, which is causing problems with our extensions that expect python to be in /usr/bin/python