ovn: support requested-chassis option for logical switch ports

ovn: support requested-chassis option for logical switch ports

This patch adds support for a "requested-chassis" option for logical
switch ports. If set, the only chassis that will claim this port is the
chassis identfied by this option; if already bound by another chassis,
it will be released.

The primary benefit of this enhancement is allowing a CMS to prevent
"thrashing" in the southbound database during live migration by keeping
the original chassis from attempting to re-bind a port that is in the
process of migrating.

This would also allow (with some additional work) RBAC to be applied
to the Port_Binding table for additional security.

Signed-off-by: Lance Richardson <lrichard@redhat.com>
Signed-off-by: Russell Bryant <russell@ovn.org>