ofctl_v1_[23]: Correct json format to send OFPExperimenter message

Because ofctl_rest inputs are restricted to json format,
this patch corrects OFPExperimenter message format of ofctl_rest.

the message body is as follows:

  experimenter  Experimenter ID. (default: 0)
  exp_type      Experimenter defined type. (default:0)
  data_type     Data encoding type. 'unicode' or 'base64'. (default: 'unicode')
  data          Experimenter-defined arbitrary additional data. (default: '')

e.g.)

  curl -X POST -d '{"dpid": 1,
                    "experimenter": 1,
                    "exp_type": 1,
                    "data": "message"}' http://localhost:8080/stats/experimenter/1

  curl -X POST -d '{"dpid": 1,
                    "experimenter": 1,
                    "exp_type": 1,
                    "data_type": "base64",
                    "data": "bWVzc2FnZQ=="}' http://localhost:8080/stats/experimenter/1

'ascii' data_type is available in ofctl_v1_[23] as until now,
but not available in REST API of ofctl_rest.

In addition, this patch corrects document of ofctl_rest.

Signed-off-by: IWASE Yusuke <iwase.yusuke0@gmail.com>