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

Cherry-pick notes:
  - Removed neutron.scheduler.l3_agent_scheduler.LeastRoutersScheduler
    from puppetdoc. It is not available in Havana.

Change-Id: I9ed4464201e1b888ae7474221babe8e5aad38642
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
(cherry picked from commit a57669a9dc3bacb80011ac45cf3da743de68e0bd)
Merge "L3 schedulers support" into stable/havana