Return a list of the images that are on the provider.

def avail_images(call=None):
    ''' Return a list of the images that are on the provider.
    '''
    if call == 'action':
        raise SaltCloudSystemExit(
            'The avail_images function must be called with '
            '-f or --function, or with the --list-images option'
        )

    items = query(method='images', root='marketplace_root')
    ret = {}
    for image in items['images']:
        ret[image['id']] = {}
        for item in image:
            ret[image['id']][item] = six.text_type(image[item])

    return ret