Create the config/provision directory after checking that config_path is writable and it doesn't exist.

Move Aegir\Provision\ServerContext\getConfigTreeBuilder to parent class for automated config building from options_documentation() method.

Add "context_name" argument to `provision status` to show information about a specific context. Add getContext() method to Aegir\Provision\Application

Add method getAllContexts() to load all context configs, and use it to list them on 'status' command.

Create Aegir\Provision\Context::save()! Try `bin/provision save server_master --remote_host=localhost`

Implement ConfigurationInterface for ServerContext, and process it in the Aegir\Provision\Context construct class.  The ConfigurationInterface provides a handy way to define and validate config values, perfect for defining our Contexts.

Adding PlatformContext, ServerContext, SiteContext classes and implementing their options_documentation in the `save` command.