uwsgi: run mapproxy as uwsgi app
app: don't mount mapproxy

If someone wants to run mapproxy it can be run via uwsgi in standalone
mode:

uwsgi --http :8080 --master --wsgi-file uwsgi/mapproxy_uwsgi.py \
      --pyargv mapserver/mapproxy/mapproxy.yaml
config: always use SKYLINES_MAP_TILE_URL, no implicit default

And add the mapproxy mount point to it, so it can be changed easily.