If port_id is set, uuid should not be used, so net_id must be optional too

[Temikus]

trafficstars

Porting over from an old Pull Request (https://github.com/fog/fog/pull/3014) so the work doesn't go to waste: https://github.com/fog/fog/pull/3014/commits/6d1ca20b521cca79a71ae62dfcd33305ad3ed8e1

I'm not that savvy in Openstack so up to you guys to decide whether this is a valid fix or not.