Allow to overwrite location of gitlab.yml config file.
requested to merge github/fork/pkgr/allow-to-change-gitlab-config-location-via-env-variable into master
Created by: crohr
Related to #6716, it would be nice if we could point to an alternate configuration file (e.g. /etc/gitlabhq/gitlab.yml in the case of a debian package for instance) just by setting an environment variable.