Use the encryption provider constants

... instead of the encryptor classes, whose direct usage has been
deprecated since Ocata.
See also Id221414d74af8413084c7935b762f93b7ce43c42 which applied
those changes to tempest.git.

Change-Id: I05b0200b80d8ae957caaa53d5006b050d2d49da6
diff --git a/barbican_tempest_plugin/tests/scenario/test_volume_encryption.py b/barbican_tempest_plugin/tests/scenario/test_volume_encryption.py
index c2033fb..b6e5412 100644
--- a/barbican_tempest_plugin/tests/scenario/test_volume_encryption.py
+++ b/barbican_tempest_plugin/tests/scenario/test_volume_encryption.py
@@ -91,8 +91,7 @@
         self.check_tenant_network_connectivity(
             server, CONF.validation.image_ssh_user, keypair['private_key'])
 
-        volume = self.create_encrypted_volume('nova.volume.encryptors.'
-                                              'luks.LuksEncryptor',
+        volume = self.create_encrypted_volume('luks',
                                               volume_type='luks')
         self.attach_detach_volume(server, volume, keypair)