Complete Configuration Management

So as to make CORD easier for operators to manage, we need a more realistic and complete OA&M story. The first step towards this goal is to standardize how services are configured. (See https://docs.google.com/spreadsheets/d/1VxNS2kYmRSb3HHD42IBwbjo9p8ukOcmwa82MG-fQ908/edit#gid=0 for current status.) All manual configuration needs to be managed by XOS. While pursuing this initial step, we should evaluate (and possibly prototype) the use of OpenConfig, which seems to be a good fit for vRouter, Fabric, and VTN. (Exploring the possibility of using OpenConfig with other services like vCDN and vSG will likely come later.)