registry: update job when migrating legacy payload
systemd: log length of unit being written
agent: use correct Job object in LoadJob task

Fix #737
Merge pull request #738 from bcwaldon/737

Carry actual unit file into Agent LoadJob task
etcd: refactor TLS configuration

Separate reading files from creating a TLS config object.
Merge pull request #734 from bcwaldon/migrate-payload-job

registry: update job when migrating legacy payload
Merge pull request #731 from bcwaldon/refactor-etcd-tls

etcd: refactor TLS configuration
registry: remove CheckJobPulse from interface
registry: use available data to determine Job.State

Remove two unnecessary etcd requests by using the data
that is already available on the Job model passed
to determineJobState.
registry: minimize requests to get Job from Registry