Return a list of the server appliances that are on the provider

def avail_images(conn=None, call=None):
    '''
    Return a list of the server appliances 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'
        )

    if not conn:
        conn = get_conn()

    ret = {}

    for appliance in conn.list_appliances():
        ret[appliance['name']] = appliance

    return ret