Mark test_subport_connectivity as unstable for now

It's failing from time to time and it's reported in the related bug.
So lets mark this test as unstable for now, until we will fix the issue
to make our gate more stable.

Change-Id: I8f9949639691294886523dfe7b7b37bd5fbce81c
Related-Bug: #1897796
diff --git a/neutron_tempest_plugin/scenario/test_trunk.py b/neutron_tempest_plugin/scenario/test_trunk.py
index 98fe6ae..8f260ea 100644
--- a/neutron_tempest_plugin/scenario/test_trunk.py
+++ b/neutron_tempest_plugin/scenario/test_trunk.py
@@ -15,6 +15,7 @@
 import collections
 
 from neutron_lib import constants
+from neutron_lib.utils import test
 from oslo_log import log as logging
 from tempest.common import utils as tutils
 from tempest.lib.common.utils import data_utils
@@ -246,6 +247,7 @@
             self._wait_for_trunk(vm.trunk)
             self._assert_has_ssh_connectivity(vm1.ssh_client)
 
+    @test.unstable_test("bug 1897796")
     @testtools.skipUnless(
         (CONF.neutron_plugin_options.advanced_image_ref or
          CONF.neutron_plugin_options.default_image_is_advanced),