Setting up multiple routing tables controlled by user_data
When having several NICs in a cloud VM, we may want to use all of them fully.
In Linux, this needs multiple routing tables, if
(a) we have several NICs in one subnet OR
(b) we want to have several default routes
This script creates these routes controlled by settings from cloud-init user_data.
enables this for case (a).
also enables case (b) by guessing the gateway address for NICs in other subnets.