Ozone : add documentation about Datanode http address

Currently Ozone's REST API uses the port 9864, all commands mentioned in OzoneCommandShell.md use the address localhost:9864.

This port was used by Datanode http server, which is now shared by Ozone. Changing this config means user should be using the value of this setting rather than localhost:9864 as in doc. The value is controlled by the config key dfs.datanode.http.address. We should document this information in OzoneCommandShell.md.