[Experiment] SR-MPLS

In this experiment, the SR-MPLS topology is configured in one direction only:
From c1, via c2 to c4, to mimic [the example depicted here](https://wiki.fd.io/view/VPP/Segment_Routing_for_MPLS).

A manual configuration is used, i.e. the VPP configuration is entered using a CLI config file defined in [vpp-startup.conf](./vpp-startup.conf) and serves as preparation for potential future integration of **StoneWork with VPP MPLS**.

## Testing SR-MPLS

1. To test the SR-MPLS, run:
$ make start-topology

2. Then go to the client and server by:
  $ docker exec -it ip4-client bash
  $ docker exec -it ip4-server bash

3. Try to ping the server from client:
client $ ping 2.2.2.2 -I tap-c1

4. Watch for incoming packets on server:
server $ tcpdump -nvvXXes 0 -i tap-c4 icmp

5. If you see incoming pings, then everything is good.

**Note**: You will not see a response on client side, since SR-MPLS is configured
in one way only.

6. Finally, to shutdown the topology run:
$ make stop-topology
