Instead of templating python code, make it static and parameterize it
with parameters.

kernel.json can pass default ERP5 url, which is (dirty) read by
ERP5kernel. Additionally ERP5kernel.py is just linked into kernel
directory, which simplifies debugging.

Note: This commit is still dirty, but it prepares ERP5kernel.py to be
      packaged, thus there is no reason to make more changes and
      cleanup.

/reviewed-on nexedi/slapos!206