Set 'default_gateway' for br-ex and br-fw-admin bridges only

Nailgun was able to set 'default_gateway' for any bridge that has GW.
Now it is set for br-ex and br-fw-admin only.

Change-Id: Icef21ca639fc056cb025eb80b18065f49d3e5f0e
Related-Bug: #1403007
Merge "Set 'default_gateway' for br-ex and br-fw-admin bridges only"