updating NetDHCPAgent doesn't work with new network_id that is associated with specific dhcp_agent_id

[Mitsuru Kanabuchi]

UPDATE_FAILED occured when update NetDHCPAgent resource.

In my understand, Neutron's behavior is:

1) create netwrok and subnet (neutron create-network, create-subnet)
2) create port to the network (neutron port-create)
3) at the same time, dhcp-agent is associated automatically by Neutron
4) 409 error will happen when adding same association (we can add other dhcp-agents if it is not associated with the Network)

I think Heat should remove old dhcp-agent before associating new dhcp-agent.
Or Heat should check same dhcp-agent existence before associating.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/69311