Add Scheduler config resource

[daktari]

resource "nomad_scheduler_config" "cfg" {
algorithm = "binpack"
        preemption {
            system_enabled = true
            batch_enabled = false
            service_enabled = false
        }
}

closes #153

[jrasell]

Hi @daktari and thanks for raising this PR. It seems the TestSchedulerConfig_basic you add is currently failing as detailed above and within Travis. Is this something you could look into fixing?

=== RUN   TestSchedulerConfig_basic
    testing.go:684: Step 0 error: config is invalid: "preemption": required field is not set