Make rkt port mapping test not exit immediately

The rkt port mapping test currently starts redis with --version, which
obviously makes redis exit again almost immediately. This means that the
container exists before the network status can be queried, and so the
test fails.

client/driver/rkt_test.go

@@ -449,7 +449,6 @@ func TestRktDriver_PortsMapping(t *testing.T) {
 		Driver: "rkt",
 		Config: map[string]interface{}{
 			"image": "docker://redis:latest",
-			"args":  []string{"--version"},
 			"port_map": []map[string]string{
 				map[string]string{
 					"main": "6379-tcp",