fallback to lxbr switch when no default controllers are available

Hi,
the attached patch is a preliminary, but already working solution to a problem we encountered while trying to bring Mininet to Debian (see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=757761).
Long story short: Debian does not have any of the default OF controllers at the moment. The idea is to detect this situation and fallback to a non-OpenFlow switch implementation. I found that lxbr switch is exactly that, but in principle it could be OpenVSwitch with learning bridges (I can implement it if you find it more appropriate). This, is I think a sensible approach, because if the controllers are not available, the switches don't work and user is not even informed why.

Consider this patch to be a proof of concept. Suggestions are welcome.

Cheers,
Tomasz