Adds nodetool as a node command

Not sure if this is contrary to how you saw ccm working or not @pcmanus, but this was pretty useful for me.

This command adds a new 'nodetool' node command. It does replicate a lot of the commands already baked into ccm, but it gives access to all the things it doesn't. So you can do things like:

    ccm node1 nodetool cfhistograms ks cf
    ccm node2 nodetool snapshot

One caveat to this is that the nodetool arguments aren't usable without escaping, because they are interpreted as ccm arguments, not nodetool arguments. Escaping with a double dash works though, and looks like:

     ccm node3 nodetool -- -pw asdf describering Keyspace1