fix(serial): Disable serial logging by default
fix(serial): Disable serial logging by default

The serial console logging causes VirtualBox to consume an immense
amount of CPU, making coreos-vagrant nearly unusable. Disabling it
by default treats the problem until VirtualBox can fix the problem
upstream.

pierreozoux/guides,leandrocp/coreos-vagrant,chourobin/weave-demos,errordeveloper/weave-demos,kikkomep/guides,pierreozoux/guides,cornelius-keller/coreos-vagrant-tinc,fgbreel/coreos-vagrant-kitchen-sink,kikkomep/guides,pierreozoux/guides,monkey-H/nap-vagrant,sjackman/coreos-vagrant,pierreozoux/guides,kikkomep/guides,glevand/coreos--coreos-vagrant,pierreozoux/guides,somespider/vm,2yoshi/docker-test,errordeveloper/weave-demos,ianblenke/coreos-vagrant-kitchen-sink,kcyeu/coreos-vagrant,kcyeu/coreos-vagrant,kikkomep/guides,pierreozoux/guides,chourobin/weave-demos,andrewrothstein/coreos-vagrant,rotespferd/coreos-vagrant,owlab-exp/coreos-vagrant-devel,oveits/coreos-vagrant,ianblenke/coreos-vagrant-kitchen-sink,monkey-H/nap-vagrant,kikkomep/guides,j0rdn/coreos-rancher-vagrant,pierreozoux/guides,lukebond/etcd-vagrant,errordeveloper/weave-demos,pessoa/guides,phamviet/coreos-vagrant-es-cluster,pierreozoux/guides,triple-i/coreos-vagrant,quantum/coreos-vagrant,pdxjohnny/coreos-vagrant,kikkomep/guides,phamviet/coreos-vagrant-es-cluster,chourobin/weave-demos,errordeveloper/weave-demos,kikkomep/guides,oveits/coreos-vagrant,chourobin/weave-demos,matsubo/coreos-vagrant-docker-compose,fgbreel/coreos-vagrant-kitchen-sink,kikkomep/guides,triple-i/coreos-vagrant,pessoa/guides,amouat/fleet-vagrant,matsubo/coreos-vagrant-docker-compose