Added basic support for hash_behaviour=merge in roles

Dict vars passed to roles are now properly merged
instead of simply overriding dict vars that are
coming from vars_files.

test/test_hash_behavior/playbook.yml

@@ -9,3 +9,8 @@
   tasks:
     - name: generate messages
       action: template src=message.j2 dest=/tmp/ansible_test_messages.out
+
+  roles:
+    - role: hash_behavior_test_role
+      messages:
+        inside_a_role: "Indeed!"