Provide an easy way to access stack outputs from the command line.

The outputs of a heat stack are useful for a variety of automation tasks, but are difficult to access using the existing heat command line client. While the Python API is relatively simple, it would also be helpful to expose the outputs in a way that would make them available via simple shell scripting. This blueprint adds an "output-list" command to display a list of available outputs and an "output-show" command to display the JSON-encoded value of a single output.