Fix unexpected notifications when using rabbitmq_policy with non-default vhosts

If not specifying vhost explicitly, `rabbitmqctl list_policy` only returns the policies in the default vhost.
In consequence of that, when using rabbitmq_policy with non-default vhost, it sends the `restart[rabbitmq-server]` notification every time when it called.

So, I've add a vhost option to `policy_exists?` to prevent unexpected notifications.