ONOS probes hosts with ARP packets with broadcast source address.

Recently a patch was merged that lets ONOS probe hosts. The probe packets use a broadcast MAC address as the source address. This is causing problems when 2 ONOS-controlled networks interconnect, because each will learn a host with a broadcast MAC address from the other ONOS network. This is also interfering with the host learning/probing that we do in routing applications, where we have an IP address configured on external-facing interfaces. Probe packets should use this IP address, otherwise external devices will learn about the ONOS-controlled network incorrectly.