Find a free prefix.

            Valid keys in the `args`-struct:

            * `auth` [struct]
                Authentication options passed to the :class:`AuthFactory`.
            * `args` [struct]
                Arguments for the find_free_prefix-function such as what prefix
                or pool to allocate from.

def find_free_prefix(self, args):
        """ Find a free prefix.

            Valid keys in the `args`-struct:

            * `auth` [struct]
                Authentication options passed to the :class:`AuthFactory`.
            * `args` [struct]
                Arguments for the find_free_prefix-function such as what prefix
                or pool to allocate from.
        """

        try:
            return self.nip.find_free_prefix(args.get('auth'), args.get('vrf'), args.get('args'))
        except NipapError as exc:
            self.logger.debug(unicode(exc))
            raise Fault(exc.error_code, unicode(exc))