add log driver

[lvenier]

Agent version	Docker API Version
0.34	1.43

As we are our main plugin user is working on the (flowg) [https://github.com/link-society/flowg] project I believe it would be a good feature to add the log driver in the agent and in the plugin later.

To start with I would suggest to support 2 drivers :

• json as default. No extra value :

{
  "log-driver": "json-log"
}

• syslog with the following configuration format

{
  "log-driver": "syslog",
  "log-opts": {
    "syslog-address": "udp://127.0.0.1:514"
  }
}

Don't add any other drivers or options in the first version. We'll see if the feature is adopted.