It enables that router have the external interface IP address
on setting using Neutron client. But, it isn't able to set
the external fixed IP address in Heat Template.
Let's allow the external IP address for Router in Heat Template, too.
It is needed by creating the VPN network using Heat Template, etc.
Implements: blueprint specify-router-ext-ip-template
Change-Id: I585c97b828a7c43e04e2ad4c63ddfc587cfcc4fe