cluster: Fix default namespace to work with GCE
cluster: Fix default namespace to work with GCE

Google Compute Engine networks can't have underscores or uppercase
letters in their name.

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