Iperf3 support

[teto]

Richard managed to use iperf3 with DCE as presented here:
https://groups.google.com/forum/#!searchin/ns-3-users/iperf3/ns-3-users/Wqe5dRWL6tM/ogVURAS9AAAJ
Code available https://github.com/RichardWithnell/iperf/tree/dce

This is a "low hanging fruit" (i.e. easy to do) so I open this issue in case any DCE newcomer is interested.

[thehajime]

thanks @teto and @RichardWithnell for this feature.

it would be the best to implement with bakeconf.xml as it allows to use the code of @RichardWithnell transparently.