1)Adding 'image_is_advanced' option to tempest config, so you can
run specific tests with images that are not cirros.
2)Adding decorator to run 'subport_connectivity' test with such
image for VLAN aware VM feature.
3)Configuring gate to run trunk test with ubuntu image only
4) Updating release notes and TESTING.rst with the change
Change-Id: Ie15b0e79c6ea320322b2815fb8afbc8ec95f853a