Fixed docker-cleanup-pipeline

Change-Id: I9366d754f65a2459ac1e19892ad28603b8dac53a
diff --git a/docker-cleanup-pipeline.groovy b/docker-cleanup-pipeline.groovy
index 710b1bb..388793a 100644
--- a/docker-cleanup-pipeline.groovy
+++ b/docker-cleanup-pipeline.groovy
@@ -28,7 +28,7 @@
           """, false)
     }
     stage("Run docker system prune"){
-      salt.cmdRun(pepperEnv, 'I@jenkins:slave', "docker system prune -f")
+      salt.cmdRun(pepperEnv, 'I@jenkins:slave', "docker system prune -f", false) // dont verify the result
     }
   }
 }