fix gitlab-ci init.d script for debian

This commit addresses an issue with the gitlab-ci init.d script on Debian based systems.
This script makes use of `sudo` which raise two problems on Debian:

1. by default `sudo` is not installed on Debian,
2. even if `sudo` is installed, `root` is not a sudoer by default on Debian

As a result `service gitlab_ci start` or `service gitlab_ci stop` fails throwing the following error: `root is not in the sudoers file.  This incident will be reported.`.

This commit replaces the following:

    # Switch to the app_user if it is not he/she who is running the script.
    if [ "$USER" != "$app_user" ]; then
      sudo -u "$app_user" -H -i $0 "$@"; exit;
    fi

by

    # Switch to the app_user if it is not he/she who is running the script.
    if [ "$USER" != "$app_user" ]; then
      su "$app_user" -l -c "$0 $@"; exit;
    fi

Signed-off-by: Remi BARRAQUAND <dev@remibarraquand.com>
Signed-off-by: Charles EYNARD <charles.eynard@yeastlab.fr>