remove useless default parameter

Change-Id: I3e67d55887bfce2766b941c4e66ec180ffb6c9f7
diff --git a/src/com/mirantis/mk/SaltModelTesting.groovy b/src/com/mirantis/mk/SaltModelTesting.groovy
index 200a053..7c478dd 100644
--- a/src/com/mirantis/mk/SaltModelTesting.groovy
+++ b/src/com/mirantis/mk/SaltModelTesting.groovy
@@ -8,7 +8,7 @@
  * @param testDir             directory of model
  */
 
-def setupAndTestNode(masterName, extraFormulas="linux", testDir) {
+def setupAndTestNode(masterName, extraFormulas, testDir) {
   def saltOpts = "--retcode-passthrough --force-color"
   def common = new com.mirantis.mk.Common()
   def workspace = common.getWorkspace()
@@ -22,6 +22,9 @@
     imageFound = false
   }
 
+  if (extraFormulas == "")
+    extraFormulas = "linux"
+
   img.inside("-u root:root --hostname=${masterName}") {
     wrap([$class: 'AnsiColorBuildWrapper']) {
       if (!imageFound) {