Building the virtual L3 NSE

To build the vL3 NSE, clone this repo and checkout this branch:

$ mkdir -p $GOPATH/src/github.com/cisco-app-networking
$ git clone https://github.com/cisco-app-networking/nsm-nse

Build the vL3 NSE:
```
$ ORG=myuser TAG=foo make docker-vl3
```
- The result is an image called myorg/vl3_ucnf-nse:foo