create single database instance and got unexpectedly demoted

[gaoxueyu]

I create a single database instance and patroni works by default even if there is only one instance.
But sometimes, the behavior is unexected. The detailed scenario is as following.

1. Use the HDD disk and try to make it busy.
2. The patroni process could not connected with etcd if disk is busy.
3. At this point, Patroni believes that an exception has occurred with the database, so it will perform a demotion operation.
   Actually, in the case of a single-node setup, we can avoid such a misjudgment without HA intervention, or by default,the maintenance mode should be turned on.