cloudformation's disable_rollback must be a bool

passing "yes" to boto causes severe problems.

cloud/cloudformation

@@ -196,7 +196,7 @@ def main():
             template_parameters=dict(required=False, type='dict', default={}),
             state=dict(default='present', choices=['present', 'absent']),
             template=dict(default=None, required=True),
-            disable_rollback=dict(default=False),
+            disable_rollback=dict(default=False, type='bool'),
             tags=dict(default=None)
         )
     )