Fall back to OVSBridge if no controller is available for default switch

@bocon13 @cdburkard @thinred
This merges in #351 

Perhaps this can be more beautiful, but I think the functionality is basically OK.

As noted, this should allow Mininet to work when a controller is not available.

I'm not sure whether we should also consider falling back to the Linux bridge, but for now it's just OVS in bridge mode.