attaching a purged PolicyRoute to an interface that is not purged results in an error

Created by: sanderr

Attaching a PolicyRoute for which purged == true to an interface with purged == false does add the ExtraConfig for the policy to the interface (interfaces <iface> policy route <policy_route_name>). This fails because the policy does not exist: it's config has not been deployed because purged == true. I believe the implementation that adds the ExtraConfig (ifacePolicyRoute) should do so iff self.policy.purged == false.