Remove useless decorator for _validate_policy
Remove useless decorator for _validate_policy

Every places calling _validate_policy have handled "context",
so the decorator of _validate_policy itself can be removed.

Change-Id: I20202a0b6aaff771d79b4b66ce6493255e6b735e

stackforge/senlin,openstack/senlin,openstack/senlin,stackforge/senlin,openstack/senlin