Add support for NSX/NVP Metadata services

This is a feature patch (2 of 3) that adds support for
Metadata services provided by the NSX (aka NVP) platform.

It also implements the handling of port events so that
dhcp and metadata configuration in NSX/NVP is updated
if port attributes such as fixed_ips and device_id are
updated.

Partial-implements blueprint nsx-integrated-services

Change-Id: Id2b9125b49c0e15e717605ec6ba3dea5d32ee755
Merge "Add support for NSX/NVP Metadata services"