Allow binding locally to different IP than endpoint IP

Sometimes you want the actual endpoint IP to be a load balanced IP, but
the bind IP to be a local IP (different per node).

This change allows that by using attributes from the [network-api-bind]
namespace.

Implements blueprint increase-ip-binding-flexibility

Change-Id: I4e134a5dc24ace58a7b9cb2705e66a7b17fd2554