Add tests to cover segments

This patch set adds tests to cver the neutron segments API [0].
Test lists, gets, creates, updates, and deletes segments.
Part of "Increase Neutron RBAC Coverage" initiative [1]

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

Story: 2002641
Task: 22306
Change-Id: I8eef0dcd5355d6212ead2bcb109f23ca0b613109
4 files changed