network: Fix mock OVSDB handling of sets
network: Fix mock OVSDB handling of sets

Before, our mock OVSDB client simply used the first element of the set
when inserting sets.

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