ovs_base: Fix debug-agent config for Q_USE_PROVIDERNET_FOR_PUBLIC=True

Set up external_network_bridge correctly for the case
of Q_USE_PROVIDERNET_FOR_PUBLIC=True.

This is an oversight in commit 6a633fd024347aade777ecd6545fa3efde5a959c.

Closes-Bug: #1394826
Change-Id: I33f0fe15bafb7071d8a09899d636471f49031606
Merge "ovs_base: Fix debug-agent config for Q_USE_PROVIDERNET_FOR_PUBLIC=True"