Fix loading default values from environment variables

Use cast of environment variable to expected type

Change-Id: I192772599b46545597c785bb5cc3c74f6ea5ad80
diff --git a/tcp_tests/tests/environment/test_ironic_standalone.py b/tcp_tests/tests/environment/test_ironic_standalone.py
index 7d20c7c..8d60279 100644
--- a/tcp_tests/tests/environment/test_ironic_standalone.py
+++ b/tcp_tests/tests/environment/test_ironic_standalone.py
@@ -33,7 +33,7 @@
 
         Before using, please set the correct roles and timeout:
 
-            export ROLES=["ironic_master",]
+            export ROLES='["ironic_master"]'
             export BOOTSTRAP_TIMEOUT=900
 
         , and unset these variables after the bootstrap is completed.