refactor(registry): Return boot ID from GetJobTarget

No callers of Registry.GetJobTarget need a full MachineState object. Rather
than create a dummy object, GetJobTarget can simply return the boot ID.
Merge pull request #311 from bcwaldon/refactor-GetJobTarget

refactor(registry): Return boot ID from GetJobTarget
refactor(requirements): Move requirements-parsing to JobPayload

SystemdUnitFile.Requirements is actually fleet business-logic, so it
belongs with in JobPayload.
refactor(payload): Move fleet option constants to job pkg

The meaning of the X-Fleet parameters belongs to Payload, so the
constants should be defined in the job package.
doc(job): improve comment
test(payload): clarify failure message in test