plot function does not work properly if the prefix is set with relative path.

Hi,

I set my prefix to a relative path by using beat config set command. I can run the experiment but When I try to plot the result I get this error, it looks in the wrong directory.

retrieving `plotters/plotterparameters/plot/isoroc/1'...
Traceback (most recent call last):
  File "/home/zmostaani/miniconda3/envs/beat_mini/bin/beat", line 11, in <module>
    sys.exit(main())
  File "/home/zmostaani/miniconda3/envs/beat_mini/lib/python3.6/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/home/zmostaani/miniconda3/envs/beat_mini/lib/python3.6/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/home/zmostaani/miniconda3/envs/beat_mini/lib/python3.6/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/zmostaani/miniconda3/envs/beat_mini/lib/python3.6/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/zmostaani/miniconda3/envs/beat_mini/lib/python3.6/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/zmostaani/miniconda3/envs/beat_mini/lib/python3.6/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/home/zmostaani/miniconda3/envs/beat_mini/lib/python3.6/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/home/zmostaani/miniconda3/envs/beat_mini/lib/python3.6/site-packages/beat/cmdline/decorators.py", line 75, in _decorator
    value = view_func(*args, **kwargs)
  File "/home/zmostaani/miniconda3/envs/beat_mini/lib/python3.6/site-packages/beat/cmdline/experiments.py", line 783, in plot
    output_folder
  File "/home/zmostaani/miniconda3/envs/beat_mini/lib/python3.6/site-packages/beat/cmdline/experiments.py", line 535, in plot_impl
    data.as_dict()[the_data], output_name, None, indentation + 2, format_cache)
  File "/home/zmostaani/miniconda3/envs/beat_mini/lib/python3.6/site-packages/beat/cmdline/plotters.py", line 238, in plot_impl
    with open(output_name, "wb") as fh:
FileNotFoundError: [Errno 2] No such file or directory: 'beat.zmostaani.prefix/prefix/beat.zmostaani.prefix/prefix/experiments/zmostaani/tutorial/eigenface/1/atnt-eigenfaces-75-comp-bis-vbox_roc.png'

in this example the my prefix value is "prefix": "beat.zmostaani.prefix/prefix" and I am running the experiment on the parent folder of beat.zmostaani.prefix.

I tried with different relative paths and I get the similar error, however if I use the absolute path everything is ok.