Add global environment variables to sample

I wanted to configure  but following the "SetEnvironmentVariable.yml" file in the demo folder does not work for Global Properties. So maybe completing this more complete demo sample with some global env var can be useful (it would have been for me because it would have prevent me to search into the issues if environment vars can be set :-)

demos/jenkins/jenkins.yaml

@@ -3,6 +3,12 @@ jenkins:
   numExecutors: 5
   scmCheckoutRetryCount: 2
   mode: NORMAL
+  
+  globalNodeProperties:
+    - envVars:
+        env:
+          - key: FOO
+            value: BAR
 
   clouds:
     - docker: