Return "jenkins" when BUILD_USER variable is absent

Change-Id: If3ccb6e937fc8579104ab297052fb18772689ca8
Related-PROD: PROD-31298
diff --git a/src/com/mirantis/mk/JenkinsUtils.groovy b/src/com/mirantis/mk/JenkinsUtils.groovy
index 94869eb..4513f2d 100644
--- a/src/com/mirantis/mk/JenkinsUtils.groovy
+++ b/src/com/mirantis/mk/JenkinsUtils.groovy
@@ -49,7 +49,7 @@
 def currentUsername() {
     username = ''
     wrap([$class: 'BuildUser']) {
-        username = BUILD_USER
+        username = env.BUILD_USER_ID ?: 'jenkins'
     }
     if (username) {
         return username