Fix loading default values from environment variables

Use cast of environment variable to expected type

Change-Id: I192772599b46545597c785bb5cc3c74f6ea5ad80
diff --git a/tcp_tests/templates/ironic_standalone/underlay.yaml b/tcp_tests/templates/ironic_standalone/underlay.yaml
index ab9a9d2..6f54227 100644
--- a/tcp_tests/templates/ironic_standalone/underlay.yaml
+++ b/tcp_tests/templates/ironic_standalone/underlay.yaml
@@ -76,7 +76,7 @@
 
         nodes:
           - name: {{ HOSTNAME_IRONIC }}
-            role: salt_master
+            role: ironic_master
             params:
               vcpu: !os_env SLAVE_NODE_CPU, 2
               memory: !os_env SLAVE_NODE_MEMORY, 2048