[c-nsp] Setting relay agent IP on 4500

[Nathan Lannine]

>> The issue is the 4500 stamps the relay agent IP in the DISCOVER as
>> being the incoming interface IP where the DISCOVER was received,

Yeah, that's expected and required behavior.  I sort of assumed, as
Mr. Mayer indicated, that the "global" option would still set the
giaddr to the receiving vrf gateway address, but set the source to
some address in the global table.  It never occurred to me that a dhcp
server would not support this behavior.

We accomplish this with intervrf routing and leave out the "global"
option, which keeps the discovers/requests in the vrf and sourced from
the vrf gateway address.