[Tempest] Running Trunk test with advanced image only

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
diff --git a/neutron/tests/tempest/config.py b/neutron/tests/tempest/config.py
index 5a9ac77..6830b18 100644
--- a/neutron/tests/tempest/config.py
+++ b/neutron/tests/tempest/config.py
@@ -30,6 +30,10 @@
                 default=[],
                 help='List of network types available to neutron, '
                      'e.g. vxlan,vlan,gre.'),
+    cfg.BoolOpt('image_is_advanced',
+                default=False,
+                help='Image that supports features that cirros does not, like'
+                     ' Ubuntu or CentOS supporting advanced features'),
 ]
 
 # TODO(amuller): Redo configuration options registration as part of the planned