added workspace provisioning feature

I agree with Daniel. This is a bit of a hack, but not too bad as long as it's documented.

I think the right way to solve this is for the control here to talk to Mesos slave (when Jenkins slave is offline) and get the path to the workspace. I haven't looked into the workings of WorkspaceBrowser, but given a remote hostname and remote path, can this browser show the contents? If yes, we could send the workspace path (even when the workspace is inside the sandbox) via status update and use that here.