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
Revert "ML2 plugin should not delete ports on subnet deletion"

This reverts commit 0d131ff0e9964cb6a65f64809270f9d597c2d5d1

There is really no problem with this change. However, it is probably
triggering a port_update notification to the agent for each port
with an allocated IP.
The agent handles that notification in a way which might be improved
from a scalability perspective.

I don't actually want this change to removed, I am just checking
whether neutron without it passess jobs.

Change-Id: I5494b607127b261043dcddfdc10c93a28ec20af5
Related-Bug: 1253896
Related-Bug: 1254236
Merge "Revert "ML2 plugin should not delete ports on subnet deletion""
Merge "Add support for NSX/NVP DHCP services"