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
Merge pull request #310 from bcwaldon/relocate-requirements

Move payload-related business logic into job package