Changes in the way runner works:
* start up with just an enabled flag - override is turned off by default
* Everything is based off of this i.e. there's no need to set variables
  or evaluate override etc. if enabled is false
* Removed the switched_override_from method as this becomes
  redundant since override is evaluated at trace initialization
  time i.e. on a trace by trace basis
* simplified override evaluation logic
* percolated changes to config/tracer