neutron_vpnaas_agent: kolla_docker parameter 'volumes' is wrong

In task "Running Neutron vpnaas bootstrap container",
parameter "volumes" of kolla_docker should be
"{{ neutron_vpnaas_agent.volumes }}"
NOT
"{{ neutron_vpnaas_agent.image }}"

Change-Id: Id7c95d4d6f53dfab384e387951eecf4d87459de
Closes-Bug: #1683669

ansible/roles/neutron/tasks/bootstrap_service.yml

@@ -55,7 +55,7 @@
       BOOTSTRAP:
     name: "bootstrap_neutron_vpnaas_agent"
     restart_policy: "never"
-    volumes: "{{ neutron_vpnaas_agent.image }}"
+    volumes: "{{ neutron_vpnaas_agent.volumes }}"
   when:
     - neutron_vpnaas_agent.enabled | bool
     - neutron_vpnaas_agent.host_in_groups | bool