Prevent floating-ip and ex-gateway ports should prevent net deletion

Prevent floating-ip and ex-gateway ports should prevent net deletion

bug 1044331

Old behavior meant that any port with device owner starting with
"network:" would be auto-deleted when a network was deleted.  We don't
want that behavior for floating IPs or external network gateways.
This provides a model where we explicitly list the set of owners that
should be auto-deleted.

- Also clean up NetworkInUse message to no longer mention 'attachment',
since that is API v1 terminology.

Change-Id: Icb727ae86d490456ec1ebc55cec1bf98ae6490c9