systemd: unexport systemdUnitManager.UnitsDir
systemd: create lsUnitsDir & pkg.ListDirectory
systemd: add hashUnitFile(s)
systemd: preload unit hashes in NewSystemdUnitManager
Merge pull request #987 from bcwaldon/720

Preload systemdUnitManager hash cache
unit: drop old legacy payload code
Merge pull request #990 from bcwaldon/drop-legacy-code

unit: drop old legacy payload code
registry: get unit state from new location
registry: drop legacy state code from EtcdRegistry.statesByMUSKey
agent: compare unit hashes when reconciling

This incorporates hashes into the decision when agent reconciler is
calculating what tasks should be performed.
squash: beef up testing in TestGetUnitState
Merge pull request #991 from bcwaldon/get-new-state

Stop reading from legacy state path
fleetctl: require scheme when using unix domain socket
Merge pull request #985 from bcwaldon/982

fleetctl: require scheme when using unix domain socket
*: clean up for go vet
travis: install go vet
test: run go vet
Merge pull request #999 from bcwaldon/govet

go vet
agent: immediately load (and possibly launch) differing units
Merge pull request #866 from jonboulle/866_agent_reconciler

fleet agent does not compare contents of units in reconciler