Disable br-ex (or equivalent) to be created on compute nodes

If $bridge_uplinks contains a value then an ovs bridge (usual default is br-ex) will be created. It's not necessarily a blocker besides the confusion it might generate. 
The external bridge is only needed on a "networker" node (or either a controller if in "all in one") and absolutely not necessary on computes nodes.

I suppose if one wants to create uplink bridge (and therefore l3 agent) on every node that would be a different scenario we'll have to handle - Thinking about multi l3 agents (the one replacing nova network multi agent)...

I've tested this.