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
(cherry picked from commit c7baaa068ed1d3c8b02717232edef60ba1b655f6)
Merge "Allow reading a tenant router's external IP" into proposed/juno