Add subcommand to force-delete and restore server

nova has a builtin DeferredDelete extension, which provide support for server restore and deferred deletion, but novaclient still lacks support for this. And there's already a draft implementation for this, see:
https://github.com/rackerlabs/os_deferred_delete_python_novaclient_ext/blob/master/os_deferred_delete_python_novaclient_ext/__init__.py

I'll do some works to integrate this to official python-novaclient, which means nova will add two subcommands: force-delete, restore.