Attributes file

We manage some 'snow flake' servers and it's important to specify attributes such as EBS volume ids at create time. I added support for loading attributes from a JSON file.

    knife ec2 server create --json-attributes-file=nodes/my_node.json

The attributes file loader will then pull out "normal" attributes. It could be extended to merge all different attributes or something, but this solved my immediate need.

Feedback? Thoughts?