Fix the value of delete-stack

Change-Id: I9926f03f9fce92c0e0b962a84c372f6be92eba6b
diff --git a/jjb/cloud-infra/cloud-infra-periodic-onap.yaml b/jjb/cloud-infra/cloud-infra-periodic-onap.yaml
index b69a4bf..e04d975 100644
--- a/jjb/cloud-infra/cloud-infra-periodic-onap.yaml
+++ b/jjb/cloud-infra/cloud-infra-periodic-onap.yaml
@@ -38,7 +38,7 @@
           deploy-scenario-version: master
           deploy-scenario-refspec: master
           disabled: false
-          delete-stack: yes
+          delete-stack: 'yes'
       - 'master':
           nordix-branch: master
           onap-version: master
@@ -48,7 +48,7 @@
           deploy-scenario-version: master
           deploy-scenario-refspec: master
           disabled: false
-          delete-stack: no
+          delete-stack: 'no'
 
     distro:
       - ubuntu1804