[c-nsp] slow convergence on edge BGP routers

[Pete Templin]

Shift the traffic before you drop the connection:

Advertise your routes with the right mix of communities to get ISP 1 to 
prefer ISP2 instead of your direct link. That'll cause their propagation 
of your announcement to withdraw in a more graceful manner.
Likewise, depreference the routes learned from ISP 1 so EDGE1 prefers 
the path that EDGE2 has.

Probably takes 8-15 minutes of advance work to save ~15 seconds of 
outage, but therefore probably worthwhile.