Add stop/pause/start actions for provider applications; implement for mock provider; unify provider application controls

This pull request adds start/stop/pause functionality, and implements it for the mock provider.  A provider's `provider_application` method simply needs to implement the appropriate methods.  If a method is not implemented, and the user somehow manages to start/stop/pause an app, the controller will throw an error.

In addition, this pull request adds the available_actions method to provider_applications.  By default, the only action available is TERMINATE; the mock provider implementation also allows for PAUSE/STOP/START, depending on the current state.  The new `views/provider_applications/_controls.html.haml` partial checks the available_actions and displays the relevant buttons accordingly.  All providers can use this same partial, allowing for easier maintenance.