[fix][wf] manage success build result in wf scenario

to avoid slack reporting null issues

Change-Id: I90ffc04e9916923be3347998147b2323c0b46399
us: https://mirantis.jira.com/browse/PRODX-5084
diff --git a/src/com/mirantis/mk/Workflow.groovy b/src/com/mirantis/mk/Workflow.groovy
index aa38c0c..7706859 100644
--- a/src/com/mirantis/mk/Workflow.groovy
+++ b/src/com/mirantis/mk/Workflow.groovy
@@ -325,6 +325,8 @@
                 currentBuild.result = 'FAILURE'
             }
             println "Failed jobs: ${failed_jobs}"
+        } else {
+            currentBuild.result = 'SUCCESS'
         }
 
         if (slackReportChannel) {