GitLab

- Download them on SR build and pass info to instance
- Instance prepares to process them as jinja2 templates
- Instance hooks the files into configuration location as appropriate

Every file so far is renamed *.erb -> *.in and a header added showing
that this file is autogenerated with links about what was the base
gitlab and/or omnibus version and omnibus reference revision this
template was last updated for.

So far all result configuration files are invalid - because ERB syntax
is there. We will convert the configuration files to proper jinja2
syntax and to using slapos parameters incrementally in the upcoming
patches.

NOTE (again): md5 sums are not yet fixed - we will fix them in the end
    of gitlab patches series after applying all tweaking changes.

/cc @kazuhiko, @jerome