WIP - First attempt to support relative urls.

Changes needed:

• gitlab.yml: files/gitlab-cookbooks/gitlab/templates/default/gitlab.yml.erb
• gitlab-shell-config.yml: files/gitlab-cookbooks/gitlab/templates/default/gitlab-shell-config.yml.erb
• unicorn.rb: files/gitlab-cookbooks/gitlab/templates/default/unicorn.rb.erb
• nginx-gitlab-http.conf: files/gitlab-cookbooks/gitlab/templates/default/nginx-gitlab-http.conf.erb
• Add default template value: files/gitlab-cookbooks/gitlab/attributes/default.rb
• Add value to gitlab-config-template: files/gitlab-config-template/gitlab.rb.template
• Add documentation
• Add entry in Changelog (once everything is in place and commits rebased)
• Build and test everything works

Questions:

1. In gitlab-shell.yml it is stated that the api endpoint Should end with a slash. In omnibus gitlab-shell-config.yml.erb this is not the case. Maybe it's a deprecated rule?
2. Should I also change application.rb? A user that tried these modifications didn't mention anything about this file.

\cc @jacobvosmaer