Neutron-dynamic-routing has the bgp_4byte_asn extension which allows
4byte ASNs to be used, increasing the range of valid AS numbers. Since
we cannot easily tell beforehand whether that extension is available,
allow for the maximal interval to be used by clients and let the API
validate the input.
Change-Id: Ib4695272784b4a5ebbcb792cfec82dac3ef6f3cf