L3 schedulers support

Icehouse brings new schedulers for virtual routing.
This patch aims to:
- add router_scheduler_driver flag support
- document which schedulers we could use

Change-Id: I9ed4464201e1b888ae7474221babe8e5aad38642
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge "L3 schedulers support"