Initialise host manager when starting tsdbrelay

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Description

Fixes a bug that was introduced with #2439 which now requires the initialisation of the HostManager on startup. That was forgotten to add to the tsdbrelay command, causing it to panic on startup.

Fixes: #2475

Type of change

From the following, please check the options that are relevant.

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

How has this been tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• [x] Tested manually since the fix is part of main()
• [x] Automated tests for the HostManagers as added previously

Checklist:

• [x] This contribution follows the project's code of conduct
• [x] This contribution follows the project's contributing guidelines
• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [ ] ~Any dependent changes have been merged and published in downstream modules~