Add missed common definition for mergeEnv function

Change-Id: I0cbd106b5d32714a9530ecf2dab22e828818adf3
diff --git a/src/com/mirantis/mk/Common.groovy b/src/com/mirantis/mk/Common.groovy
index 215cf7b..249175b 100644
--- a/src/com/mirantis/mk/Common.groovy
+++ b/src/com/mirantis/mk/Common.groovy
@@ -846,6 +846,7 @@
  *  @param extraVars - Multiline YAML text with extra vars
  */
 def mergeEnv(envVar, extraVars) {
+    def common = new com.mirantis.mk.Common()
     try {
         def extraParams = readYaml text: extraVars
         for(String key in extraParams.keySet()) {