Allow setting a tenant router's external IP

Adds an external_ip option to the router creation
and update operations to set the IP address the router
will try to use as its fixed IP on the external network
it's attached to. By default this is restricted to an
admin-only operation by policy.json.

DocImpact
ApiImpact

Implements: blueprint specify-router-ext-ip
Closes-Bug: #1188427
Change-Id: Iba7c606eea48181fc10e9d0d5dc667e6f48f37de
Merge "Allow setting a tenant router's external IP"