Attaching a pre-existing port with port security_disabled on a network with port_security enabled fails

[Gaudenz Steinlin]

Description
===========

Attaching a pre-existing port with port security_disabled on a network with port_security enabled which does not have any subnets fails. The port_security setting on the network should not be relevant in this case. It's only a default value for newly created port. For pre-existing ports the port_security setting on the port should be considered instead.

This fails because there is code to prohibit attaching to a network with port_security enabled which does not have a subnet. Because then it's not possible to attach security groups to the port. This is correct in case a port is actually created by Nova and the port_security set on the network is applied for the created port, but it's wrong for already existing ports. The port_security setting on the port should be considered instead.

Steps to reproduce
==================

* Create an instance
* Create a network with port security enabled
* Create a port on this network with port security disabled
* Try to attach the port to the instance

Note: No subnet was created on the network.

Expected result
===============

The port is attached to the instance.

Actual result
=============

The port fails to attach to the instance with this message:

Network requires port_security_enabled and subnet associated in order to apply security groups. (HTTP 400) (Request-ID: req-3ce456bb-c016-4737-82f8-4b332b923ab6)

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/nova/+/912478