Return a list of all cache subnet group names

    CLI example::

        salt myminion boto_elasticache.list_subnet_groups region=us-east-1

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

    CLI example::

        salt myminion boto_elasticache.list_subnet_groups region=us-east-1
    '''
    return [g['CacheSubnetGroupName'] for g in
            get_all_cache_subnet_groups(name, region, key, keyid, profile)]