This patch rehomes neutron's external-net API definition into
neutron-lib. It also includes the related exception. While this
exception is not used externally today [1], it seems anyone implementing
this extension may need to use it.
UTs and a release note are also included.
[1] http://codesearch.openstack.org/?q=ExternalNetworkInUse
Change-Id: I9933b91d1e82db3891b3b72f06e94316e56a4f15
