Add users to the 'gitlab-www' group via gitlab.rb

The documentation tells users to add there www-data user to the gitlab-www group, but Chef will kick the new group member out on every gitlab-ctl reconfigure run!

Discovered by @cshulz in #263 (closed).

We need to add a setting to gitlab.rb so that administrators can add extra system users to the gitlab-www group via gitlab-ctl reconfigure.

cc @marin