renku run fails with external files

[m-alisafaee]

Describe the bug
When external files are used in reknu run an exception is raised.

To Reproduce
Steps to reproduce the behavior:

1. renku dataset add --create my-data --external /path/to/file
2. renku run cat data/my-data/file > output3.

Expected behavior
Running with external file should be the same as running with files in the project.

Screenshots and/or execution output

Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.7/bin/renku", line 8, in <module>
    sys.exit(cli())
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/click/core.py", line 1137, in __call__
    return self.main(*args, **kwargs)
  File "/Users/Me/Documents/Workspace.noindex/sdsc/code/renku-python/renku/cli/exception_handler.py", line 128, in main
    result = super().main(*args, **kwargs)
  File "/Users/Me/Documents/Workspace.noindex/sdsc/code/renku-python/renku/cli/exception_handler.py", line 92, in main
    return super().main(*args, **kwargs)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/click/core.py", line 1062, in main
    rv = self.invoke(ctx)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/click/core.py", line 1668, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/click/core.py", line 763, in invoke
    return __callback(*args, **kwargs)
  File "/Users/Me/Documents/Workspace.noindex/sdsc/code/renku-python/renku/cli/run.py", line 321, in run
    command_line=command_line,
  File "/Users/Me/Documents/Workspace.noindex/sdsc/code/renku-python/renku/core/management/command_builder/command.py", line 283, in execute
    output = context["click_context"].invoke(self._operation, *args, **kwargs)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/click/core.py", line 763, in invoke
    return __callback(*args, **kwargs)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/inject/__init__.py", line 342, in injection_wrapper
    return sync_func(*args, **kwargs)
  File "/Users/Me/Documents/Workspace.noindex/sdsc/code/renku-python/renku/core/commands/run.py", line 157, in _run_command
    **{name: os.path.relpath(path, working_dir) for name, path in mapped_std.items()},
  File "/Users/Me/Documents/Workspace.noindex/sdsc/code/renku-python/renku/core/management/workflow/plan_factory.py", line 124, in __init__
    self.add_inputs_and_parameters(*detected_arguments)
  File "/Users/Me/Documents/Workspace.noindex/sdsc/code/renku-python/renku/core/management/workflow/plan_factory.py", line 240, in add_inputs_and_parameters
    default_value=self._path_relative_to_root(default.path),
  File "/Users/Me/Documents/Workspace.noindex/sdsc/code/renku-python/renku/core/management/workflow/plan_factory.py", line 646, in _path_relative_to_root
    return str((self.directory / path).resolve().relative_to(self.working_dir))
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/pathlib.py", line 900, in relative_to
    .format(str(self), str(formatted)))
ValueError: '/Users/Me/Documents/Workspace.noindex/sdsc/data/LEGO.csv' does not start with '/private/tmp/a'