Fixes #21293: Allow overriding rudder-web.properties parameters with files in rudder-web.properties.d

[fanf]

https://issues.rudder.io/issues/21293

Add the use of an overriding configuration directory. By default, it is the name of the config file with d. appended to it but can be overridden if needed with a JVM parameter.

Log info for such things are important, they looks like:

[2022-06-20 10:42:57+0200] INFO  application - Use configuration file defined by JVM property -Drudder.configFile : /home/fanf/java/workspaces/rudder-project/static-config/configuration-rudder-7.1.properties
[2022-06-20 10:42:57+0200] INFO  application - JVM property -Drudder.configDir is not defined: using /home/fanf/java/workspaces/rudder-project/static-config/configuration-rudder-7.1.properties.d directory
[2022-06-20 10:42:57+0200] DEBUG application - Overriding configuration files in '/home/fanf/java/workspaces/rudder-project/static-config/configuration-rudder-7.1.properties.d': 10-low-override.properties, 90-high-override.properties
[2022-06-20 10:42:57+0200] DEBUG application - loading configuration from FileSystemResource(/home/fanf/java/workspaces/rudder-project/static-config/configuration-rudder-7.1.properties.d/90-high-override.properties)
[2022-06-20 10:42:57+0200] DEBUG application - loading configuration from FileSystemResource(/home/fanf/java/workspaces/rudder-project/static-config/configuration-rudder-7.1.properties.d/10-low-override.properties)
[2022-06-20 10:42:57+0200] DEBUG application - loading configuration from FileSystemResource(/home/fanf/java/workspaces/rudder-project/static-config/configuration-rudder-7.1.properties)
[2022-06-20 10:42:57+0200] DEBUG application - Writing resolved configuration file to /home/fanf/java/workspaces/rudder-project/static-config/configuration-rudder-7.1.properties.d/rudder-web.properties-resolved-debug

The rudder-web.properties-resolved-debug file is a debbuging file with origin info looking like:

...
            },
            # merge of /.../configuration-rudder-7.1.properties.d/90-high-override.properties,/.../configuration-rudder-7.1.properties
            "reportscleaner" : {
                # /.../configuration-rudder-7.1.properties.d/90-high-override.properties
                "archive" : {
                    # /.../configuration-rudder-7.1.properties.d/90-high-override.properties
                    "TTL" : "2"
                },
                # /.../configuration-rudder-7.1.properties
                "compliancelevels" : {
                    # /.../configuration-rudder-7.1.properties
                    "delete" : {
                        # /.../configuration-rudder-7.1.properties
                        "TTL" : "8"
                    }
                },
...

[fanf]

PR updated with a new commit

[fanf]

PR updated with a new commit

[fanf]

PR updated with a new commit

[Normation-Quality-Assistant]

This PR is not mergeable to upper versions.
Since it is "Ready for merge" you must merge it by yourself using the following command:
rudder-dev merge https://github.com/Normation/rudder/pull/4337
-- Your faithful QA
Kant merge: "It is beyond a doubt that all our knowledge begins with experience."
(https://ci.normation.com/jenkins/job/merge-accepted-pr/56443/console)

[fanf]

OK, squash merging this PR