ovn-northd: Add hint in lflow to link back to acl

ovn-northd: Add hint in lflow to link back to acl

It will be helpful for trouble-shooting if we can link a logical flow
back to the ACL that generated it. This patch is to add a stage-hint as
an external-id in lflow. The hint contains stage specific information.
Now only lflows in ACL stages have hint, which is the ACL uuid, though
the same mechanism can be used to add hint for other stages later.

Signed-off-by: Han Zhou <zhouhan@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>