File-based secrets aren't supported

[mlowicki]

https://github.com/mesosphere/marathon/blob/master/docs/docs/secrets.md#file-based-secrets

  File "/Users/mlowicki/.mgrve/lib/python2.7/site-packages/marathon/models/base.py", line 43, in from_json
    return cls(**{to_snake_case(k): v for k, v in attributes.items()})
  File "/Users/mlowicki/.mgrve/lib/python2.7/site-packages/marathon/models/app.py", line 113, in __init__
    else MarathonContainer.from_json(container)
  File "/Users/mlowicki/.mgrve/lib/python2.7/site-packages/marathon/models/base.py", line 43, in from_json
    return cls(**{to_snake_case(k): v for k, v in attributes.items()})
  File "/Users/mlowicki/.mgrve/lib/python2.7/site-packages/marathon/models/container.py", line 44, in __init__
    for v in (volumes or [])
  File "/Users/mlowicki/.mgrve/lib/python2.7/site-packages/marathon/models/base.py", line 43, in from_json
    return cls(**{to_snake_case(k): v for k, v in attributes.items()})
TypeError: __init__() got an unexpected keyword argument 'secret'