Use the encryption provider constants (2)

Follow-up of I05b0200b80d8ae957caaa53d5006b050d2d49da6
which fixes also the other leftover usage of a legacy
encryptor class (plain this time).

Change-Id: I8fc4e2e6ed84bf18a4c89c30ee5c40b743d00e46
diff --git a/barbican_tempest_plugin/tests/scenario/test_volume_encryption.py b/barbican_tempest_plugin/tests/scenario/test_volume_encryption.py
index 9241283..26919c6 100644
--- a/barbican_tempest_plugin/tests/scenario/test_volume_encryption.py
+++ b/barbican_tempest_plugin/tests/scenario/test_volume_encryption.py
@@ -133,7 +133,6 @@
         self.check_tenant_network_connectivity(
             server, CONF.validation.image_ssh_user, keypair['private_key'])
 
-        volume = self.create_encrypted_volume('nova.volume.encryptors.'
-                                              'cryptsetup.CryptsetupEncryptor',
+        volume = self.create_encrypted_volume('plain',
                                               volume_type='cryptsetup')
         self.attach_detach_volume(server, volume, keypair)