[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
diff --git a/neutron/tests/tempest/scenario/test_trunk.py b/neutron/tests/tempest/scenario/test_trunk.py
index 5d775ef..1412b11 100644
--- a/neutron/tests/tempest/scenario/test_trunk.py
+++ b/neutron/tests/tempest/scenario/test_trunk.py
@@ -18,6 +18,7 @@
 from tempest.lib.common.utils import data_utils
 from tempest.lib import decorators
 from tempest import test
+import testtools
 
 from neutron.common import utils
 from neutron.tests.tempest.common import ssh
@@ -221,6 +222,9 @@
                                 CONF.validation.image_ssh_user,
                                 self.keypair['private_key'])
 
+    @testtools.skipUnless(
+          CONF.neutron_plugin_options.image_is_advanced,
+          "Advanced image is required to run this test.")
     @decorators.idempotent_id('a8a02c9b-b453-49b5-89a2-cce7da66aafb')
     def test_subport_connectivity(self):
         vlan_tag = 10