Add high availability tool for neutron networking

* Setup optional cron jobs to call the tool in order to find
offline l3 agents and migrate them to another online agent
* Supports a rebalance feature to shuffle tenants around
given the scheduler for neutron is very simplistic right now
* Supports rebalancing DHCP configuration to all DHCP agents

Change-Id: Iaf0686c2fa93ec1dcdd644b75cb3b4528d4cabf0