[j-nsp] VPLS label allocation

[Phill Jolliffe]

Need to be careful when describing the formula. local/remote site id
means varies depending on perspective.

for example:

PE-A, (connected to customer site id 1), advertises a label base of
1000, range of 4 and offset of 1

This route is received by remote PE-B that has another sight in the
same VPN. The site connected to PE-B is site id 3.

Traffic forward from site 3 to site 1 will arrive at PE-A with a label 1002

label base + remote sight id - offset