[Neutron] Reduce DHCP agent redundancy

Issue

In environments with many networks, the DHCP agent re-sync process can take a long time. Every time a DHCP agent is restarted, it will need to request from the Neutron server all the networks, subnets and ports information. That could take several minutes (and in some cases, more than one hour).

In HA systems, the redundancy value defined by default is 3 (dhcp_agents_per_network=3). That means each network will be handled by three DHCP agents. Once a new network has been created, the DHCP scheduler will assign it to the configured number, creating a "networkdhcpagentbindings" database register per network and DHCP agent. This binding can be only removed automatically if the DHCP is considered dead.