Fix an issue where the ordering of an include statement is processed ahead of…

Fix an issue where the ordering of an include statement is processed ahead of the conditional in the included statement,
which required a redundant check to see if a variable was defined rather than short circuiting.

Fixes #4469

lib/ansible/playbook/task.py

@@ -274,6 +274,6 @@ class Task(object):
             self.only_if = utils.compile_when_to_only_if(self.when)
 
         if additional_conditions:
-            self.only_if = [ self.only_if ] 
-            self.only_if.extend(additional_conditions)
-
+            new_conditions = additional_conditions
+            new_conditions.append(self.only_if)
+            self.only_if = new_conditions