[Boron] Openflow plugin deletes switch from topology while changing mastership from one controller to another

A controller that holds mastership to a switch and lose connectivity delete the switch from the topology even though there are other controllers which will valid connectivity and another controller takes ownership.

Reproducing this error is very straightforward:

• Start a 3 controllers nodes
• connect a switch to all controllers
• delete the controller that holds mastership from the switch

Then, switch disappears from the topology and appears again when another controllers takes ownership.