Implementations should be able to specify the identifier type to use for unknown patients

As an implementer, I want to discourage the use and promulgation of temporary identifiers. This becomes very difficult when temporary identifiers are indistinguishable from preferred identifiers.

If I could specify the identifier type to be used when registering an unknown patient, I could create easily distinguish temporary identifiers from proper/preferred identifiers. This becomes important once the unknown patient is merged into a proper record, after which we want to make it very easy to discriminate between temporary identifiers (which should not be promulgated on paper or external systems) and proper identifiers (we do this by making our temporary identifier format noticeably different with a leading "X" or "TEMP").

Acceptance Criteria

1. an administrator can (optionally) set a global property (probably "registration.unknownPatient.identifierSource" to point to a specific identifier source from the idgen module.
2. if this GP is set, creating an Unidentified patient will pull from this identifier source, and not assign a normal OpenMRS ID.
3. if this GP is not set, creating an Unidentified patient generates an ID just like a normal patient.