neutron v2 ports APIs allow to list ports by security group already:
https://docs.openstack.org/api-ref/network/v2/#show-port-details

This patch adds the `SecurityGroups` field to `ListOpts`.

One way to filter the ports by security group can be done with the
following code:

```
listOpts := ports.ListOpts{
	SecurityGroups: []string{"2183457b-70cc-4fd0-a2dc-95323fa19e45"}
}

allPages, err := ports.List(networkClient, listOpts).AllPages()
if err != nil {
	panic(err)
}

allPorts, err := ports.ExtractPorts(allPages)
if err != nil {
	panic(err)
}

for _, port := range allPorts {
	fmt.Printf("%+v\n", port)
}
```