Datanode config separation (#15701)

* Different variants of security configuration (from file, from mongo, insecure).
Encryption/decryption pattern used in practice.

* Reverting temporary changes

* More precise certificate presence check

* Added unit tests for testHasCert() method

* Datanode and opensearch configuration refactored

* code cleanup

* code cleanup

* code cleanup

* Node preflight config service refactored

* restbaseurl moved to OpensearchConfiguration

---------

Co-authored-by: Lukasz Kaminski <lukasz.kaminski@graylog.com>

data-node/src/main/java/org/graylog/datanode/Configuration.java

@@ -87,7 +87,7 @@ public class Configuration extends BaseConfiguration {
     private String configLocation;
 
     @Parameter(value = "process_logs_buffer_size")
-    private Integer logs = 500;
+    private Integer opensearchProcessLogsBufferSize = 500;
 
 
     @Parameter(value = "node_name")
@@ -172,7 +172,7 @@ public class Configuration extends BaseConfiguration {
     }
 
     public Integer getProcessLogsBufferSize() {
-        return logs;
+        return opensearchProcessLogsBufferSize;
     }
 
     @Parameter(value = "rest_api_username")