engine: Warning message for no workers/no masters
engine: Warning message for no workers/no masters

Added warning message for users that (1) don't specify a master
machine but specify worker machine(s) or (2) don't specify worker
machine(s) but specify a master machine. Also removes the canBoot
function.

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