Need to make VSPERF's MoonGen codebase in parity with master emmerich MoonGen branch.

Today VSPERF uses a Lua script named opnfv-vsperf.lua to perform MoonGen RFC 2544 testing. The repository for obtaining this MoonGen code is here:

https://github.com/atheurer/MoonGen

which several months ago was forked from Paul Emmerich's MoonGen github:

https://github.com/emmericp/MoonGen

Given the dynamic state of this repository, it has been a challenge to keep parity between the two git codebases. Moving forward - to ensure parity - instead of using a fork to maintain code, MoonGen / VSPERF will use submodules. To be specific, instead of opnfv-vsperf.lua, there will be a new script called trafficgen.lua in a new github location. This new repository will have trafficgen.lua provide the RFC 2544 support and will download https://github.com/emmericp/MoonGen as submodules.