Add command override to via msg.command

[macinspak]

Added a simple way to use msg.command to override the configured command on the command node.

[sammachin]

This needs to work as I mentioned in the issue comment if I'm going to merge it #5 (comment)

as I said in the issue I don't believe that fixed input params are a good design pattern for node-red and it needs to be a typed input.

There also needs to be validation that the input is a valid command for the selected cluster an if not an appropriate error