Reimplement DHCPAgent as net's property

We implemented Neutron's DHCPAgent resource as Heat's NetDHCPAgent resource at
once.

  https://review.openstack.org/#/c/59626/

However, we discussed appropriate way of how to implement L3Agent in the
following review:

  https://review.openstack.org/#/c/69303/

The conclusion is to implement Neutron's L3Agent as router's property.
We should refactor NetDHCPAgent in the same way as RouterL3Agent.

Change-Id: I7a0118ed4445b31186e24c545d4529867ce78c6c
Closes-bug: #1294965
Merge "Reimplement DHCPAgent as net's property"