Bump neutron to 0.3.15

This sync asap2 patch to community work
diff --git a/roles/neutron/vars/main.yml b/roles/neutron/vars/main.yml
index a4cd93f..47aff4d 100644
--- a/roles/neutron/vars/main.yml
+++ b/roles/neutron/vars/main.yml
@@ -62,6 +62,8 @@
           tunnel_id_ranges: 1:1000
         ml2_type_vlan:
           network_vlan_ranges: external:1:4094
+    netoffload:
+      enabled: true
   manifests:
     ingress_server: false
     service_ingress_server: false