Make sure DHCP agent manifest gets added
Re-work L2 plugin/agent + quantum for multi-node

Since every agent requires quantum.conf, and we bundle that in the
openstack-quantum package along with all of the quantum-server
stuff, anywhere we run an agent we also have to install the quantum
API server.

This patch does that, but makes sure the service is only enabled on
the API server. It also splits out the l2 plugin config from the
agent config.