Skip unstable v6 scenario tests

In the interest of stabilizing the gate closer to the end of Newton, this
test should be skipped as it fails sporadically showing that IPv6 is not
robust enough. No-one is taking serious ownership of the required fix,
and thus the onus is on IPv6 owners to re-enable this test should they
care about the feature being rock solid, and continuously tested.

Related-bug: #1540983

Change-Id: Ibd30f8d90b1ce76a219e02935cec52b402b15168
diff --git a/tempest/scenario/test_network_v6.py b/tempest/scenario/test_network_v6.py
index 59ebb7a..364b6f5 100644
--- a/tempest/scenario/test_network_v6.py
+++ b/tempest/scenario/test_network_v6.py
@@ -18,6 +18,7 @@
 
 from tempest import config
 from tempest.lib.common.utils import test_utils
+from tempest.lib import decorators
 from tempest.scenario import manager
 from tempest import test
 
@@ -254,6 +255,7 @@
         self._prepare_and_test(address6_mode='dhcpv6-stateless', n_subnets6=2,
                                dualnet=True)
 
+    @decorators.skip_because(bug="1540983")
     @test.idempotent_id('9178ad42-10e4-47e9-8987-e02b170cc5cd')
     @test.services('compute', 'network')
     def test_dualnet_multi_prefix_slaac(self):