Add datanode information to support bundle

[moesterheld]

Description

This adds initial information about data nodes to the support bundle.
Information from the data nodes as stored in mongodb are provided in datanodes.configured and status information from the status endpoint of running data nodes is provided in datanodes.running in the generated cluster.json

Additionally, the status endpoint now return information about the operating system, java version and user running the data node as well as free disk space available for the configured data node directories.

Motivation and Context

resolves https://github.com/Graylog2/graylog-plugin-enterprise/issues/7966

How Has This Been Tested?

manually by creating a support bundle in System/Logging

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[todvora]

Tested locally, looks good, just one question