Add support for neutron-designate ingtegration

Contrail shall support neutron-designate integration as per https://docs.openstack.org/mitaka/networking-guide/config-dns-int.html

Use cases for neutron integration are:

Basic work flows:
- Ability to assign dns_name attribute to ports
- Ports are assigned a dns_name and dns_assignment based on VM name

For external DNS service (designate):
Use case 1: Ports are published directly in the external DNS service
Use case 2: Floating IPs are published with associated port DNS attributes
Use case 3: Floating IPs are published in the external DNS service