Removed latest SALT_VERSION passing to test salt formula pipeline

Change-Id: I161083f1764c04bd9be83a185dbf3af4f9e035dc
diff --git a/test-salt-formulas-pipeline.groovy b/test-salt-formulas-pipeline.groovy
index 56e9bb9..3c75950 100644
--- a/test-salt-formulas-pipeline.groovy
+++ b/test-salt-formulas-pipeline.groovy
@@ -105,7 +105,7 @@
         try {
           saltVersion = SALT_VERSION
             } catch (MissingPropertyException e) {
-          saltVersion = "latest"
+          saltVersion = "" // default value is empty string, means latest
         }
         withEnv(["SALT_VERSION=${saltVersion}"]) {
           sh("make clean && make test")