rkt: create parent cgroup to enable stats

Having the Nomad executor create parent cgroups that rkt is launched
within allows the stats collection code used for the exec driver to Just
Work. The only downside is that now the Nomad executor's resource
utilization counts against the cgroups resource limits just as it does
for the exec driver.

client/driver/rkt.go

@@ -194,7 +194,10 @@ func rktGetDriverNetwork(uuid string, driverConfigPortMap map[string]string, log
 					return nil, lastErr
 				}
 
-				// This is a successful landing.
+				// This is a successful landing; log if its not the first attempt.
+				if try > 1 {
+					logger.Printf("[DEBUG] driver.rkt: retrieved network info for pod UUID %s on attempt %d", uuid, try)
+				}
 				return &cstructs.DriverNetwork{
 					PortMap: portmap,
 					IP:      status.Networks[0].IP.String(),
@@ -211,7 +214,7 @@ func rktGetDriverNetwork(uuid string, driverConfigPortMap map[string]string, log
 		}
 
 		waitTime := getJitteredNetworkRetryTime()
-		logger.Printf("[DEBUG] driver.rkt: getting network info for pod UUID %s failed attempt %d: %v. Sleeping for %v", uuid, try, lastErr, waitTime)
+		logger.Printf("[DEBUG] driver.rkt: failed getting network info for pod UUID %s attempt %d: %v. Sleeping for %v", uuid, try, lastErr, waitTime)
 		time.Sleep(waitTime)
 	}
 	return nil, fmt.Errorf("timed out, last error: %v", lastErr)
@@ -665,9 +668,13 @@ func (d *RktDriver) Start(ctx *ExecContext, task *structs.Task) (*StartResponse,
 		return nil, fmt.Errorf("failed to set executor context: %v", err)
 	}
 
+	// Enable ResourceLimits to place the executor in a parent cgroup of
+	// the rkt container. This allows stats collection via the executor to
+	// work just like it does for exec.
 	execCmd := &executor.ExecCommand{
-		Cmd:  absPath,
-		Args: runArgs,
+		Cmd:            absPath,
+		Args:           runArgs,
+		ResourceLimits: true,
 	}
 	ps, err := execIntf.LaunchCmd(execCmd)
 	if err != nil {
@@ -818,7 +825,7 @@ func (h *rktHandle) Kill() error {
 }
 
 func (h *rktHandle) Stats() (*cstructs.TaskResourceUsage, error) {
-	return nil, DriverStatsNotImplemented
+	return h.executor.Stats()
 }
 
 func (h *rktHandle) run() {

client/driver/rkt_test.go

@@ -94,7 +94,6 @@ func TestRktDriver_Start_DNS(t *testing.T) {
 	}
 
 	ctestutils.RktCompatible(t)
-	// TODO: use test server to load from a fixture
 	task := &structs.Task{
 		Name:   "etcd",
 		Driver: "rkt",