Add missed common package definition for generateModel function

Change-Id: I5ed90ca7b21155f9e788388fdb2b046bcf01a132
diff --git a/src/com/mirantis/mk/Python.groovy b/src/com/mirantis/mk/Python.groovy
index d5dd5fc..c3b2c97 100644
--- a/src/com/mirantis/mk/Python.groovy
+++ b/src/com/mirantis/mk/Python.groovy
@@ -279,6 +279,7 @@
  * @return
  */
 def generateModel(context, contextName, saltMasterName, virtualenv, modelEnv, templateEnvDir, multiModels = true) {
+    def common = new com.mirantis.mk.Common()
     def generatedModel = multiModels ? "${modelEnv}/${contextName}" : modelEnv
     def templateContext = readYaml text: context
     def clusterDomain = templateContext.default_context.cluster_domain