Add OS::Neutron::NetDHCPAgent for neutron/net.py

This resource supports part of "agent management and scheduler
extensions" in Neutron.  It associates network with dhcp-agent.

I think, it may be needed by administrators who want to decide agents to
work. (e.g. to use HA functionality)

Closes-bug: #1245366
Change-Id: I3dc663c4c51824bd3e69c74ff1d9b50a8ce598c9
Merge "Add OS::Neutron::NetDHCPAgent for neutron/net.py"