Set onlink routes for all subnets on an external network

The addition of the on-link routes gives us some freedom to allocate a
router's IP address from any one of multiple subnets on one external
network.  Different routers can get their IPs from different subnets and
they still have direct on-link connectivity to each other.  For example,
one router with its primary IP from 10.0.0.0/24 and another from
192.168.0.0/24 can communicate directly.  It is important that each
router has on-link routes to *all* of the subnets.

Any router can host floating ips from any of the subnets regardless of
which subnet the primary IP address comes from.

This is an alternative to the "Multiple floating IP pools" section in
the administration guide.  It is a simpler alternative that avoids
having to create multiple external networks.  It is also more flexible
because routers will no longer be restricted to getting floating IPs
from the pool to which they happen to be connected.

DocImpact
Document the procedure for adding subnets to the external network.
Potentially remove the existing procedure for "Multiple floating IP
pools" from the docs.

Change-Id: I2c283f5be0cbb6b5d350cafc1b636c300b796a7b
Closes-Bug: #1312467
Merge "Set onlink routes for all subnets on an external network"