Allow router external IPs to be set and read

This is a specification is to allow tenants to read
the external IP addresses assigned to their router
and for admins to explicitly set the IP addresses
on a router.

blueprint specify-router-ext-ip

Change-Id: Ibeaed1280e6736ef3b5f00db871946ad9c1e6c22
Merge "Allow router external IPs to be set and read"