Mesos 0.19.0 container support

Together with @tarnfeld 's initial effort, I patched and improved the container support feature:

* Container input box is persisted correctly
* Each slave configuration can have their own container configuration, this enables option to run dind or single docker image for different build labels/setups
* When container image option is empty, TaskInfo is built without container command
 * if mesos-slave has containerizer_path and isolation setup, it will run the default container
 * otherwise mesos-slave will run normally without containerizer. 

I tested on an AWS setup, it works nicely. 

Update: updated README for container support.

This PR should have addressed #44 and delivered #18
  
kill this. JenkinsScheduler can directly do the check whether the image is empty or not.