Adding support of DNS nameserver and Host routes for the Nuage Plugin

This commit will implement the following DHCP options:
 - DNS nameserver
 - Host routes

Implements: blueprint dhcp-host-routes-and-dns-support-for-nuage-plugin
Change-Id: Idbb04f46112cddd588cad88921210aa0f63645f6
Merge "Adding support of DNS nameserver and Host routes for the Nuage Plugin"