[sliceable_switch] fixed vlan_tag modification

Fixed vlan_tag modification in making path.

There was no actions to modify vlan_id in flow entries which
setup by this controller after "flow_manager support".
In a case which source port and dest port are in different
vlan_ids, some packets were sent properly from the controller
until flow entries are setup. After that, packets were sent
with mismatched vlan_id due to source port vlan_id was kept.

This patch adds a vlan_tag modification action to the flow
entry at the last switch on the path, so that user can receive
packets with correct vlan_id and the controller can use same
openflow_match. This is the same way to update vlan_tag which
sliceable_switch did before "flow_manager support".