Do not output anything in places used by cron (whenever).
Create gitlab_ci user with shell but with locked password.

Previously a user was created with --disabled-login, which caused the scheduled builds to fail.

The --disabled-login gives the user `/bin/false` as shell, which breaks `whenever` that tries to spawn `/bin/bash -l` in order to get the proper path for running rake task via `bundler`.
Updated upgrade doc with with fix for scheduled builds.