Use @test.services instead of skipUnless

This is to use @test.services('image') instead of
@testtools.skipUnless(CONF.service_available.glance),
just trivial fix.

Change-Id: I7241ff29c59044adfd2d890030df45cbe2e4687c
diff --git a/tempest/api/volume/test_volumes_backup.py b/tempest/api/volume/test_volumes_backup.py
index 5ad209c..9c7be70 100644
--- a/tempest/api/volume/test_volumes_backup.py
+++ b/tempest/api/volume/test_volumes_backup.py
@@ -118,9 +118,8 @@
                                     name=backup_name, force=True)
         self.assertEqual(backup_name, backup['name'])
 
-    @testtools.skipUnless(CONF.service_available.glance,
-                          "Glance is not available")
     @decorators.idempotent_id('2a8ba340-dff2-4511-9db7-646f07156b15')
+    @test.services('image')
     def test_bootable_volume_backup_and_restore(self):
         # Create volume from image
         img_uuid = CONF.compute.image_ref
diff --git a/tempest/api/volume/test_volumes_clone.py b/tempest/api/volume/test_volumes_clone.py
index a6bbb0a..4c13375 100644
--- a/tempest/api/volume/test_volumes_clone.py
+++ b/tempest/api/volume/test_volumes_clone.py
@@ -13,11 +13,10 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import testtools
-
 from tempest.api.volume import base
 from tempest import config
 from tempest.lib import decorators
+from tempest import test
 
 
 CONF = config.CONF
@@ -47,9 +46,8 @@
         self.assertEqual(volume['source_volid'], src_vol['id'])
         self.assertEqual(volume['size'], src_size + 1)
 
-    @testtools.skipUnless(CONF.service_available.glance,
-                          "Glance is not available")
     @decorators.idempotent_id('cbbcd7c6-5a6c-481a-97ac-ca55ab715d16')
+    @test.services('image')
     def test_create_from_bootable_volume(self):
         # Create volume from image
         img_uuid = CONF.compute.image_ref