compute: Correctly pass certified_image_ref as image_id

Ib346d383c430d5151d9aafa6e856dd0a7cae8a23 incorrectly attempted to pass
this as imageRef leading to duplicate arguments being used when calling
the servers client to create the server.

Closes-Bug: #1834461
Change-Id: Ic913176b429b3465dcdbc2e4f44eea32f0b17dfa
diff --git a/tempest/api/compute/servers/test_servers.py b/tempest/api/compute/servers/test_servers.py
index 91ebbc0..76d65dd 100644
--- a/tempest/api/compute/servers/test_servers.py
+++ b/tempest/api/compute/servers/test_servers.py
@@ -196,7 +196,7 @@
     def test_show_update_rebuild_list_server(self):
         trusted_certs = CONF.compute.certified_image_trusted_certs
         server = self.create_test_server(
-            imageRef=CONF.compute.certified_image_ref,
+            image_id=CONF.compute.certified_image_ref,
             trusted_image_certificates=trusted_certs,
             wait_until='ACTIVE')