helper function to find subnet explicit route table associations

    .. versionadded:: 2016.11.0

def _get_subnet_explicit_route_table(subnet_id, vpc_id, conn=None, region=None, key=None, keyid=None, profile=None):
    '''
    helper function to find subnet explicit route table associations

    .. versionadded:: 2016.11.0
    '''
    if not conn:
        conn = _get_conn(region=region, key=key, keyid=keyid, profile=profile)
    if conn:
        vpc_route_tables = conn.get_all_route_tables(filters={'vpc_id': vpc_id})
        for vpc_route_table in vpc_route_tables:
            for rt_association in vpc_route_table.associations:
                if rt_association.subnet_id == subnet_id and not rt_association.main:
                    return rt_association.id
    return None