Change the prefix for trunk subports device_owner
Change the prefix for trunk subports device_owner

The choice was poorly made as the 'network:' prefix is used extensively
in the codebase to identify ports created by the platform itself. Subports
instead are user created.

Closes-bug: 1624570

Change-Id: Ie792a154a6946d0acd5bed322363319e241b1ae7

cloudbase/neutron,sebrandon1/neutron,huntxu/neutron,sebrandon1/neutron,noironetworks/neutron,noironetworks/neutron,openstack/neutron,mahak/neutron,openstack/neutron,mahak/neutron,openstack/neutron,mahak/neutron,cloudbase/neutron,eayunstack/neutron,eayunstack/neutron,huntxu/neutron