Make host also customizable with file/environment variable

Since we have the port value configurable I think it'll be good to add such configuration options also for the host value. For example I'm running GDK inside of docker container and I need to bind gitlab-workhorse to container's external address, not the container's localhost. I could set this manually in the Procfile but this will be easier and faster with environment variable and also this will be consistent with the port value configuration method.