Changing the default value of hdds.scm.safemode.min.datanode to 3

[nandakumar131]

Currently, the default value of hdds.scm.safemode.min.datanode is set to 1.
This means that an user can deploy an Ozone cluster (with default configuration) with just one datanode and the cluster will come out of safemode. After coming out of safemode the writes with three way replication will fail.

By changing the default value of hdds.scm.safemode.min.datanode from 1 to 3, we will make sure that the cluster deployed with default configuration will not fail during write once it's out of safemode.

The users who wants to run Ozone with just one datanode can manually update the config.

[arp7]

This makes sense, cc @errose28 & @sodonnel.

[errose28]

+1. Seems to follow the principle of least surprise. Let's make sure the safemode message calls out the config when there are not enough datanodes. We should check on some of the docker quick start examples that start with only one datanode to make sure they still work out of the box.