add possibility to get output of run-cmd in json

[aabadie]

Right now, the only way to get the output is to activate the verbose mode using --verbose. This is not very convenient to get the output of a command per node. We should display them as a json field with the following structure:

{ 
  "0" : ["node1.site", "node2.site"]
  "output":
    {
      "node1.site" : "command output",
      "node2.site" : "command output"
    }
}