Add LeastRouters Scheduler to Neutron L3 Agent

Allow scheduling of a virtual router on an L3 Agent node with the least
number of routers currently scheduled. This scheduler can be used
instead of the default random scheduler.

Also refactor the l3_agent_scheduler to allow for adding new schedulers.

Implement blueprint lessrouter-scheduler
Change-Id: Ie539c08bdc8a6e1430a106f77d08f15abd0903e7
Merge "Add LeastRouters Scheduler to Neutron L3 Agent"