Add Flintrock commands for remotely executing shell commands on cluster nodes

[nchammas]

Users launching clusters often need to do their own additional setup to install packages they need or the like.

We should offer one, maybe two new Flintrock commands to make it easy for users to run commands remotely on all the nodes of their cluster. I'm imagining they will work like stripped-down, Flintrock-specific versions of pssh.

Here's a proposal for what those commands might look like:

./flintrock run-command my-cluster 'yum install -y package'
./flintrock run-script my-cluster /path/to/local/script.sh

For starters I want to keep these commands as simple as possible. Just run the shell command or script on each of the cluster nodes in parallel and report whether they all passed or not. In the event of a failure, I might not even want to show which specific node or nodes failed, at least for an initial cut of this feature.

Anything more, like getting the output back from each of the nodes, will be pushed off to later, or redirected to utility tools like pssh which are better suited to the task.

[nchammas]

I'm closing this issue since we have run-command (#26) implemented. I'm going to punt on implementing run-script since equivalent functionality can be composed by the user using copy-files (#7) and run-command.