Use new encryption provider constants (reprise)

This is a follow-up of:
https://review.opendev.org/453144

and it fixes the last usage of the encryptor classes
which was introduced by https://review.opendev.org/472219
while the previous "clean-up" change was waiting to be merged.

Change-Id: I564d9f4d603c7f0dbfdde65dac1325c26fdff999
diff --git a/tempest/scenario/test_volume_boot_pattern.py b/tempest/scenario/test_volume_boot_pattern.py
index 0782389..3b4bbda 100644
--- a/tempest/scenario/test_volume_boot_pattern.py
+++ b/tempest/scenario/test_volume_boot_pattern.py
@@ -252,8 +252,7 @@
     @utils.services('compute', 'volume')
     def test_boot_server_from_encrypted_volume_luks(self):
         # Create an encrypted volume
-        volume = self.create_encrypted_volume('nova.volume.encryptors.'
-                                              'luks.LuksEncryptor',
+        volume = self.create_encrypted_volume('luks',
                                               volume_type='luks')
 
         self.volumes_client.set_bootable_volume(volume['id'], bootable=True)