Add migration support from agent to NSX dhcp/metadata services

Add migration support from agent to NSX dhcp/metadata services

This is feature patch (3 of 3) that introduces support for
transitioning existing NSX-based deployments from the agent
based model of providing dhcp and metadata proxy services
to the new agentless based mode. In 'combined' mode, existing
networks will still be served by the existing infrastructure,
whereas new networks will be served by the new infrastructure.

Networks may be migrated to the model using a new CLI tool
provided, called 'neutron-nsx-manage'. Currently the tool
provides two admin-only commands:

  neutron-nsx-manage net-report <net-id-or-name>

This will check that the network can be migrated and returns
the resources currently in use. And:

  neutron-nsx-manage net-migrate <net-id-or-name>

This will move the network over the new model and deallocate
resources from the agent. Once a network has been migrated
there is no turning back.

Completes-blueprint nsx-integrated-services

Change-Id: I37c9aa0e76124e1023899106406de7be6714c24d