When we using tools like gpstart\gpstop\gpconfig, we rely the cluster alive to fetch cluster info from it to decide if we're in singlenode mode or cluster mode.
However, if we kill the cluster by gpstop, it's a little hard to get info from the dead service. Thus, when we use gpstart to restart the stopped singlenode service, we need to add "--singlenodeMode" to control the behavior of gpstart.

An idea is to write mode info in postgres conf file? Not sure. Support it in the future.