Distribution create command doesn't say that base_path is required

[daviddavis]

Related to #572. Looking at the options for distribution create, it says that name is required but not --base-path:

$ pulp file distribution create --help
Usage: pulp file distribution create [OPTIONS]

  Create a file distribution.

Options:
  --base-path TEXT
  --publication TEXT  Publication to be served. This will unset the
                      'repository' and disable auto-distribute.
  --repository TEXT   Repository to be used for auto-distributing. When set,
                      this will unset the 'publication'. Specified as
                      '[[<plugin>:]<type>:]<name>' or as href.
  --name TEXT         [required]
  --help              Show this message and exit.