Fix broken L3 support of Ryu plugin
Fix broken L3 support of Ryu plugin

Fixes bug 1047230

This patch adds support for the router:external attribute of a network
to Ryu plugin.

Change-Id: I135fe0a4e90fe049cabbaa1b87e9294ec0613530

gkotton/vmware-nsx,gkotton/vmware-nsx