[r=andreserl][bug=1659607][author=mpontillo] API cleanup for `interface link-subnet`.
* Fix bug that disallowed changing a LINK_UP link to a LINK_UP link with a different subnet on the same VLAN (or no subnet).
* Allow a subnet that seems invalid or a LINK_UP mode to be forcibly set via the API. (new force=True parameter)
* Allow an interface to be explicitly disconnected. (Removes all IP addresses, and unsets its VLAN.)