ofagent: Add a missing push_vlan action

Fix the flow for _provision_local_vlan_inbound_for_tunnel
to push-vlan explicitly.  While the old code happened to
work with older versions of OVS, it was spec-wise incorrect because
it failed to meet the prerequisite of the following set-field.
The latest version of OVS correctly rejects such a flow.

Closes-Bug: 1308927
Change-Id: I66221eec0cb4083d178d7d5651360ee1874e3d1b