#2723 (Oct 19, 2020, 11:19:00 PM)

gnuradio zmq broker refactoring

First step towards support everal ENBs and installing the remote script
at runtime:

* The gnuradio broker class is moved to its own file, to decouple it
  from RFemu.
* The ENBs are registered earlier in the test so that the GrBroker knows
  when to start (delay start + setup until all ENBs have been configured).
* Handle ENBs internally as a list.

Change-Id: I4f1095bbc7ed0a816fe47caef44f7becadd9d737