Set l3_agent_manager for ovs/linuxbridge plugin

related to quantum bug 1139726, L3NATAgentWithStateReport makes
no longer the default, as doing so broke DHCP for other plugins.

Quantum linuxbridge has the agent extension support. This commit also
set dhcp/l3_agent_manager to *AgentWithStateReport in linuxbridge plugin.

Change-Id: I4095f4276499468d02265169cc26bbd8489679bc