Add option to clean workspace, default to yes

vars/checkoutStep.groovy

@@ -3,10 +3,14 @@
 void call(Map config = [:], Closure body = { }) {
     Map defaults = [
         stepName: 'Checkout',
-        useJobSCM: true
+        useJobSCM: true,
+        cleanWorkspace: true
     ]
     runConfig = defaults + config
     stage(runConfig.stepName) {
+        if (runConfig.cleanWorkspace) {
+            cleanWs disableDeferredWipeout: true
+        }
         printBanner(runConfig.stepName)
         if (runConfig.useJobSCM) {
             checkout scm