docker: Don't print a log message on each Exec
docker: Don't print a log message on each Exec

The future networking patches rely heavily Exec in the ovs-vswitchd
container.  The log message per Exec becomes an annoyance in this
case.

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