Add default platform to kitchen tests main function

Change-Id: Ib78ee1b3daf5b7fcf1e4116801776989ec34010b
diff --git a/src/com/mirantis/mk/Ruby.groovy b/src/com/mirantis/mk/Ruby.groovy
index c9b7e45..fc6d102 100644
--- a/src/com/mirantis/mk/Ruby.groovy
+++ b/src/com/mirantis/mk/Ruby.groovy
@@ -37,7 +37,7 @@
 /**
  * Run kitchen tests in tests/integration
  */
-def runKitchenTests(platform){
+def runKitchenTests(platform=""){
     runKitchenCommand("converge ${platform}")
     runKitchenCommand("verify -t tests/integration ${platform}")
     runKitchenCommand("destroy ${platform}");