VOL-643: Allow user to specify their own ONOS config in the stack file

Change-Id: I8fb92a60f20b7c2156582dc43de2cb69792fe688
diff --git a/compose/voltha-stack.yml.j2 b/compose/voltha-stack.yml.j2
index 4622925..728ad5a 100644
--- a/compose/voltha-stack.yml.j2
+++ b/compose/voltha-stack.yml.j2
@@ -317,6 +317,10 @@
       ONOS_CONNECTION: "onos:8181"
       ONOS_USER: "karaf"
       ONOS_PASSWORD: "karaf"
+{%- if ONOS_CONFIG is defined and ONOS_CONFIG %}
+    volumes:
+      - {{ ONOS_CONFIG }}:/data/onos-config.json
+{%- endif %}
     networks:
       - voltha-net