Add flag to device registration validator to allow devices without a token

[joe94]

Currently, there is no way to configure whether or not to allow devices that do not present a token. We'd like to create one that's off by default but which when enabled, allows devices to still connect to the cluster with a lower level of trust.

This work depends on the validator library implementing xmidt-org/bascule#58

[joe94]

During team discussions, we found that we need further architectural discussion about device trust levels and registration.