Skip test_volume_extend_when_volume_has_snapshot until bug 1687044 is fixed

This test is still failing at a really high rate:

http://status.openstack.org/elastic-recheck/#1687044

And that's just the jobs where elastic-recheck didn't time out
waiting to get logs, so the actual failure rate is probably higher.

Let's skip it for now.

Change-Id: I49cfa97c5a2a8d30514adb59d9ad3ae35a6ade37
Related-Bug: #1687044
diff --git a/tempest/api/volume/test_volumes_extend.py b/tempest/api/volume/test_volumes_extend.py
index 837758f..1eb76a0 100644
--- a/tempest/api/volume/test_volumes_extend.py
+++ b/tempest/api/volume/test_volumes_extend.py
@@ -40,6 +40,7 @@
     @decorators.idempotent_id('86be1cba-2640-11e5-9c82-635fb964c912')
     @testtools.skipUnless(CONF.volume_feature_enabled.snapshot,
                           "Cinder volume snapshots are disabled")
+    @decorators.skip_because(bug='1687044')
     def test_volume_extend_when_volume_has_snapshot(self):
         volume = self.create_volume()
         self.create_snapshot(volume['id'])