Make VLAN-aware-VMs configurable

VLAN-aware-VMs are only supported with OVS firewall driver.

This patch makes the feature configurable and leaves it disabled
by default to ensure the feature is only available on supported
configurations to avoid confusion.

Change-Id: I65d88f7aa37f2f44f4446d9054d731defb90cd19
diff --git a/metadata/service/gateway/single.yml b/metadata/service/gateway/single.yml
index fe7b886..1af2525 100644
--- a/metadata/service/gateway/single.yml
+++ b/metadata/service/gateway/single.yml
@@ -19,6 +19,7 @@
       external_access: True
       security_groups_enabled: true
       qos: false
+      vlan_aware_vms: false
       metadata:
         host: ${_param:cluster_vip_address}
         password: ${_param:metadata_password}