Drop host_directory support

[mccalluc]

host_directory was part of the machinery for providing input via locally mounted files. We've dropped support for that.

            binding = volume.copy()
            assert not binding.get('mode'), \
                '"mode" should not be provided on {}'.format(volume)
            host_directory = binding.pop('host', None)
            if host_directory:
                # Typically for mounting user input
                binding['mode'] = 'ro'
                volumes[host_directory] = binding