Make before_script and after_script overridable
This is makes it possible to overwrite the before_script and after_script at job level.
This is continuation of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3771
This is makes it possible to overwrite the before_script and after_script at job level.
This is continuation of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3771