Use HUP to stop gerrit in docker-compose

Also add a 5 minute stop grace period. This lines up with the init
script we were using when this was a systemd managed service.

Change-Id: I5a92bb214b96447008ad570e176adda13c4ca0cb

playbooks/roles/gerrit/templates/docker-compose.yaml.j2

@@ -5,6 +5,8 @@ services:
     image: {{ gerrit_container_image }}
     network_mode: host
     user: gerrit
+    stop_signal: SIGHUP
+    stop_grace_period: 5m
     volumes:
 {% for volume in gerrit_container_volumes %}
       - {{ volume }}