use multiple neutron dhcp agent mode for ha-network solution.

In 5.1 release and before FUEL HA network solution was based on one neutron-dhcp-agent, switchable between controllers.
In 6.0 version an ability to use multiple DHCP agents instead of a single agent was added but disabled by default.
This blueprint describes the way of using this feature by default. The main idea is rather similar to multiple-l3-agents. DHCP agents are located on all controller nodes and serve Neutron networks and subnets. The HA solution is based on the networks rescheduling mechanism and the ability to schedule network to more than one agent.