Return a list of all cache subnet group names

    Example:

    .. code-block:: bash

        salt myminion boto3_elasticache.list_cache_subnet_groups region=us-east-1

def list_cache_subnet_groups(region=None, key=None, keyid=None, profile=None):
    '''
    Return a list of all cache subnet group names

    Example:

    .. code-block:: bash

        salt myminion boto3_elasticache.list_cache_subnet_groups region=us-east-1
    '''
    return [g['CacheSubnetGroupName'] for g in
            describe_cache_subnet_groups(None, region, key, keyid, profile)]