Add skip for cinder extend volume with snapshot

Some drivers [1] are unable to perform the operation of extending
volume that has a snapshot. Add a skip so this test does not get run.

[1] https://docs.openstack.org/cinder/latest/configuration/block-storage/drivers/ibm-storwize-svc-driver.html#extend-volumes

Change-Id: Ie988c9a6be396688b4d78d6a31dfffe8525b97b3

tempest/api/volume/test_volumes_extend.py

@@ -46,6 +46,9 @@ class VolumesExtendTest(base.BaseVolumeTest):
     @decorators.idempotent_id('86be1cba-2640-11e5-9c82-635fb964c912')
     @testtools.skipUnless(CONF.volume_feature_enabled.snapshot,
                           "Cinder volume snapshots are disabled")
+    @testtools.skipUnless(
+        CONF.volume_feature_enabled.extend_volume_with_snapshot,
+        "Extending volume with snapshot is disabled.")
     def test_volume_extend_when_volume_has_snapshot(self):
         """Test extending a volume which has a snapshot"""
         volume = self.create_volume()

tempest/config.py

@@ -1107,7 +1107,13 @@ VolumeFeaturesGroup = [
                      'server instance? This depends on the 3.42 volume API '
                      'microversion and the 2.51 compute API microversion. '
                      'Also, not all volume or compute backends support this '
+                     'operation.'),
+    cfg.BoolOpt('extend_volume_with_snapshot',
+                default=True,
+                help='Does the cloud support extending the size of a volume '
+                     'which has snapshot? Some drivers do not support this '
                      'operation.')
+
 ]