xcube gen doesn't work for installed plugins

[forman]

Describe the bug

xcube gen when run with configuration now fails:

  File "C:\Users\Norman\Miniconda3\envs\xcube\lib\site-packages\click\core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "C:\Users\Norman\Miniconda3\envs\xcube\lib\site-packages\click\core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "C:\Users\Norman\IdeaProjects\xcube\xcube\cli\gen.py", line 122, in gen
    **config)
TypeError: gen_cube() got an unexpected keyword argument 'output_name'

This is a regression after changes made to address #19 and #49.

To Reproduce

> xcube --traceback gen -c config.yml "data/s2plus/2017/07/*.nc"

[forman]

Not a bug: some configuration parameters have changed, that's why the error occurred, namely 'output_dir', 'output_name' --> output_path