Gitlab uses github-markup to render various text-based markups
(markdown, rst, ...) into html. For rst github-markup wants to run
python and have docutils egg available:

    https://github.com/github/markup/blob/5393ae93/lib/github/markups.rb#L36

as we were not having docutils installed and path to proper python
interpreter setup, rst documents were not automatically rendered and
were show just as plain text.

We do a lot of documents in rst - that case is important for us.

So fix it by providing gitlab with properly setup python interpreter
with all needed eggs installed.

/cc @kazuhiko, @jerome
/proposed-for-review-on nexedi/slapos!39