Add support for NSX/NVP DHCP services

This is a feature patch (1 of 3) that adds support for DHCP
services provided by the NSX (aka NVP) platform. Green-field
deployments can use the AGENTLESS mode, which will make the
core plugin interact solely with the NSX platform to provide
DHCP services. Support for metadata proxy services, migration
for brown field deployments will be added in future patches.

Partial-implements blueprint nsx-integrated-services

Change-Id: Idfc4b2d871e70cd557d8e0e6b23e5563f9ed3420
Merge "Add support for NSX/NVP DHCP services"