Re-set external_network_bridge for l3 agent

Without this option set to blank neutron will attempt to dump raw
frames which is not correct. Kevin Benton is working on fixing the
deprecation message.

Change-Id: Ib6d6f41e2f222d383583e79958a78f2a09d3ae93
Closes-Bug: #1561156

ansible/roles/neutron/templates/l3_agent.ini.j2

 # l3_agent.ini
 [DEFAULT]
 agent_mode = legacy
+external_network_bridge =