Add --override-attributes option to override node attributes for a single chef run.

A working prototype that allows overriding node attributes from the command line.

Usage Example: 
```bash
knife solo cook <node> --override-attributes '{"mysql": {"server_root_password": "<passwd>"}}'
```

I want this so I can share my kitchen and keep sensitive attributes out of the node.json file. My cookbooks only run certain resources when the attributes needed are set. (eg: only install and configure mysql when the node['mysql']['server_root_password'] is set)

The code is a little hacky, but it works. Any ideas for improvement are welcome.