Do not rely on signed images in ephemeral encryption test

The ephemeral encryption might be enabled but nova barbican integration
is not enabled.
The patch allows to use CONF.compute.image_ref in such cases.

Related-Prod: PRODX-14559
Change-Id: I8dfb5d5f32bba4f9260195157539f7b084bc7ac4
diff --git a/barbican_tempest_plugin/tests/scenario/test_ephemeral_disk_encryption.py b/barbican_tempest_plugin/tests/scenario/test_ephemeral_disk_encryption.py
index 3734019..3d1808c 100644
--- a/barbican_tempest_plugin/tests/scenario/test_ephemeral_disk_encryption.py
+++ b/barbican_tempest_plugin/tests/scenario/test_ephemeral_disk_encryption.py
@@ -46,7 +46,10 @@
     def test_encrypted_ephemeral_lvm_storage(self):
         test_string = 'Once upon a time ...'
         client_test_path = '/tmp/ephemeral_disk_encryption_test'
-        img_uuid = self.sign_and_upload_image()
+        if CONF.image_signature_verification.enforced:
+            img_uuid = self.sign_and_upload_image()
+        else:
+            img_uuid = CONF.compute.image_ref
         keypair = self.create_keypair()
         security_group = self._create_security_group()
         instance = self.create_server(