Fixing typo for cert_validate

Due to this typo cert_validate had been recognized as tuple but
not boolean.

Change-Id: I379ddaf644d199e3c1b609d31e28942fb1a1f12f
diff --git a/heat_tempest_plugin/tests/api/test_heat_api.py b/heat_tempest_plugin/tests/api/test_heat_api.py
index 38df5b8..d256437 100644
--- a/heat_tempest_plugin/tests/api/test_heat_api.py
+++ b/heat_tempest_plugin/tests/api/test_heat_api.py
@@ -93,7 +93,7 @@
             else:
                 register_test_case_id(test_case)
 
-    cert_validate = not conf.disable_ssl_certificate_validation,
+    cert_validate = not conf.disable_ssl_certificate_validation
     try:
         api_tests = driver.build_tests(test_dir, loader, url=endpoint, host="",
                                        fixture_module=fixtures,