[1] Added a tests which requires "segments" plugin but
we don't have it enabled in overcloud. It is enabled in
neutron jobs from long[2]. This patch adds this plugin
to the default enabled plugins.
[1] https://review.openstack.org/#/c/558609/
[2] https://review.openstack.org/#/c/459439/
Related-Bug: #1765008
Change-Id: I1bc36c4533dcaadd81d7c93a194e9319217c69cd