Return a list of the image sizes that are on the provider

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

    items = query(method='sizes', command='?per_page=100')
    ret = {}
    for size in items['sizes']:
        ret[size['slug']] = {}
        for item in six.iterkeys(size):
            ret[size['slug']][item] = six.text_type(size[item])

    return ret