Allow reading a tenant router's external IP

Adds an external IPs field to the external gateway information
for a router so the external IP address of the router can be
read by the tenant.

DocImpact

Closes-Bug: #1255142
Change-Id: If4e77c445e9b855ff77deea6c8df4a0b3cf249d4
Merge "Allow reading a tenant router's external IP"