don't override systemd override.conf

debian/postinst

@@ -266,7 +266,11 @@ case "$1" in
       for service in mailroom unicorn sidekiq workhorse; do
         path=/etc/systemd/system/gitlab-${service}.service.d
         mkdir -p $path
+        if [ -e $path/override.conf ]; then
+          echo "$path/override.conf already exist" 
+        else
           printf "[Service]\nUser=${gitlab_user}\n" > $path/override.conf
+        fi
       done
 
       # Manage gitlab-shell's config.yml via ucf