gitlab: Fix rendering of ReStructuredText documents

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 !39

software/gitlab/software.cfg

@@ -31,6 +31,7 @@ parts =
     pkgconfig
     nginx-output
 
+    python-4gitlab
     gitlab-shell/vendor
     gitlab/vendor/bundle
     gitlab-workhorse
@@ -53,6 +54,17 @@ parts =
 #   Software compilation   #
 ############################
 
+# python with eggs, that will be used in gitlab
+[python-4gitlab]
+# NOTE cannot use zc.recipe.egg - github-markup invoks `python2 -S` and
+# interpreter generated by zc.recipe.egg cannot handle that.
+recipe  = z3c.recipe.scripts
+# NOTE github-markup invokes it as `python2`, that's why we are naming it this way
+# https://github.com/github/markup/blob/5393ae93/lib/github/markups.rb#L36
+interpreter = python2
+eggs    =
+    docutils
+
 # rubygemsrecipe with fixed url and this way pinned rubygems version
 [rubygemsrecipe]
 recipe  = rubygemsrecipe
@@ -81,8 +93,10 @@ bundle  = ${buildout:bin-directory}/bundle
 # execjs needs: nodejs
 # rails needs db client program on path: psql
 # gitlab wants to check redis version via running: redis-cli
+# gitlab (via github-markup) wants to convert rst -> html via running: python2 (with docutils egg)
+# (python-4gitlab puts interpreter into ${buildout:bin-directory})
 environment =
-  PATH    = ${:ruby-location}/bin:${cmake:location}/bin:${pkgconfig:location}/bin:${nodejs:location}/bin:${postgresql92:location}/bin:${redis28:location}/bin:%(PATH)s
+  PATH    = ${:ruby-location}/bin:${cmake:location}/bin:${pkgconfig:location}/bin:${nodejs:location}/bin:${postgresql92:location}/bin:${redis28:location}/bin:${buildout:bin-directory}:%(PATH)s
 
 
 # gitlab, gitlab-shell & gitlab-workhorse checked out as git repositories
@@ -266,6 +280,8 @@ md5sum  = 9bdca16362fe19c727bca38383e57068
 
 [versions]
 cns.recipe.symlink = 0.2.3
+docutils = 0.12
 plone.recipe.command = 1.1
 rubygemsrecipe  = 0.2.2
 slapos.recipe.template = 2.9
+z3c.recipe.scripts = 1.0.1