Use the proper condition for volume encryption transfer tests

Like all other volume encryption tests (with attachment or boot
from volume), make sure that also the volume encryption transfer
tests are executed only when the appropriate configuration key
(compute_feature_enabled.attach_encrypted_volume) is set.

Change-Id: I21014b915dbecd0abaabb214f18f5241cc7c5b47
diff --git a/cinder_tempest_plugin/scenario/test_encrypted_volume_transfer.py b/cinder_tempest_plugin/scenario/test_encrypted_volume_transfer.py
index 7232433..bfe33b2 100644
--- a/cinder_tempest_plugin/scenario/test_encrypted_volume_transfer.py
+++ b/cinder_tempest_plugin/scenario/test_encrypted_volume_transfer.py
@@ -41,6 +41,8 @@
     @classmethod
     def skip_checks(cls):
         super(TransferEncryptedVolumeTest, cls).skip_checks()
+        if not CONF.compute_feature_enabled.attach_encrypted_volume:
+            raise cls.skipException('Encrypted volume attach is not supported')
         if not CONF.service_available.barbican:
             raise cls.skipException('Barbican is required')