Implement mechanism to supply user data defaults to CloudMan.

This will allow for image specific CloudMan options (e.g. paths or default_bucket_url) to specified without requiring every user to specify these parameters via user data at launch time. Values specified by user at launch/runtime will take precedence.

These image specific parameters are specified as a file. By default installed_files/image_user_data will be used. Image specific properties can be added directly to this file or a completely new file may be specified by setting image_user_data_path or image_user_data_template_path in fabricrc.

Submitting this as a pull request because it changes CloudMan's behavior somewhat and I would like to provide the opportunity for comment. I think as more people build custom images this will become an important feature so I would be happy to adjust things as you guys see fit. I will merge this soon unless there are objections.