Add tests to cover trunks subports

This patch set adds tests to cover the neutron subports API [0].
Test adds, gets, removes trunks subports.
Part of "Increase Neutron RBAC Coverage" initiative [1]

[0] https://developer.openstack.org/api-ref/network/v2/index.html#add-subports-to-trunk
[1] https://storyboard.openstack.org/#!/story/2002641

Change-Id: I5ddefe2df23b4888ce6706a27784c461f9af1614
Story: 2002641
Task: 22328
1 file changed