cli: Consistent box name parameter name (BOX_NAME) in task descriptions.
Pulled --debug class_option up to parent class.
cli: Add class_option --cwd to change the working directory.
cli: Moved some provider independent task methods (templates, list) to parent class.
cli: Make provider registration information (name and usage) available to provider class and instance.
cli: Moved common task methods into parent class
     (destroy, halt, up, ssh, winrm, copy, define, undefined, ostypes, sendkeys)
Fixed config file loading from (optional) workdir.
Added alias -w for the --cwd option.
Fixed and improved provider registration. Removed duplicate code.
Merge pull request #562 from Drachenfels-GmbH/master

more cli code deduplication, improved provider registration & easy workdir switching