feature(agent,machine): track job resource requirements and machine specs

On the agent level, remember what each job declares for resource requirements.
This will be queried from systemd in the future.
This PR does not yet publish the free space on the machine to etcd yet.
codereview feedback
more codereview work
Merge pull request #441 from uwedeportivo/machineload

feature(agent,machine): track job resource requirements
style(*): ran gofmt -s and made golint changes

* omitting type annotations for literals when inferrable from context
* (`gofmt -s`)
* omitting second `range` key when ignored (`k, _` -> `k`, `gofmt -s`)
* Idiomatic error messages: Start lowecase, except where referencing an
* uppercase value/type, no terminal punctuation (`golint`)
* `errors.New(fmt.Sprintf(args))` -> `fmt.Errorf(args)` (`golint`)
* `if foo { return bar } else { baz }` -> `if foo { return bar } baz`
* (`golint`)

Conflicts:
	fleetctl/ssh_test.go