ovn: Add OVN northbound API
ovn: Add OVN northbound API

This patch adds support for queries to the OVN northbound database. In
particular, it exposes an API for fetching, creating, and deleting
logical switches and stateless ACLs.

mmussomele/quilt,tejasmanohar/quilt,NetSys/quilt,mmussomele/quilt,quilt/quilt,PenguinToast/quilt,kelda/kelda,PenguinToast/quilt,tejasmanohar/quilt,NetSys/quilt,kklin/quilt,kklin/kelda,ejj/quilt-dev,ZhongtianWang/quilt,stevenlin598/quilt,secant/quilt,ZhongtianWang/quilt,LuiseV/quilt,quilt/quilt,LuiseV/quilt,ejj/quilt-dev,kayousterhout/quilt,kklin/kelda,stevenlin598/quilt,ejj/quilt-dev,akshayknarayan/quilt,PenguinToast/quilt,stevenlin598/quilt,mmussomele/quilt,secant/quilt,mmussomele/quilt,PenguinToast/quilt,kayousterhout/quilt,kelda/kelda,secant/quilt,kayousterhout/quilt,secant/quilt,mmussomele/quilt,kklin/quilt,akshayknarayan/quilt,NetSys/quilt,kklin/quilt,LuiseV/quilt,quilt/quilt,tejasmanohar/quilt,ZhongtianWang/quilt,stevenlin598/quilt,mmussomele/quilt,kelda/kelda,stevenlin598/quilt,kklin/kelda,secant/quilt,NetSys/quilt,quilt/quilt,kklin/kelda,NetSys/quilt,stevenlin598/quilt,tejasmanohar/quilt,kayousterhout/quilt,ejj/quilt-dev,PenguinToast/quilt,akshayknarayan/quilt,kelda/kelda,LuiseV/quilt,akshayknarayan/quilt,kklin/quilt,ZhongtianWang/quilt,LuiseV/quilt