ipatests: update expected error message

With commit ec6698f , the error message has changed from
  Unable to communicate with CMS (503)
to
  Request failed with status 503: Non-2xx response from CA REST API: 503.  (503)

Related: https://pagure.io/freeipa/issue/8704
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>

ipatests/test_integration/test_ipahealthcheck.py

@@ -436,7 +436,8 @@ class TestIpaHealthCheck(IntegrationTest):
         error_msg = (
             "Request for certificate failed, "
             "Certificate operation cannot be completed: "
-            "Unable to communicate with CMS (503)"
+            "Request failed with status 503: "
+            "Non-2xx response from CA REST API: 503.  (503)"
         )
         returncode, data = run_healthcheck(
             self.master, "ipahealthcheck.dogtag.ca",