Use ebtables to isolate dhcp traffic

Iptables doesn't properly block the broadcast traffic crossing
the bridge, so use ebtables instead. Removes test which is no
longer valid since we are not using iptables anymore.

Change-Id: I43e5f1fe9512dd3ec9595c7203bc46837cef3cad
Closes-Bug: #1318104
Merge "Use ebtables to isolate dhcp traffic"