Removing rails secrets facilities from RMT

[josegomezr]

Description

RMT does not use any secret facility (key derivation for singing or encrypting data), yet rails requires a valid configuration for this to boot fine.

This is now removed and hardcoded a dummy secret key base in case any other component within the rails stack tries to derivate a key.

Further helps #1064

Change Type

• [x] Bug Fix (a non-breaking change which fixes an issue)
• [ ] New Feature (a non-breaking change which adds new functionality)
• [ ] Documentation Update (a change which only updates documentation)

Checklist

• [x] I have verified that my code follows RMT's coding standards with rubocop.
• [x] I have reviewed my own code and believe that it's ready for an external review.
• [x] I have provided comments for any hard-to-understand code.
• [x] I have documented the MANUAL.md file with any changes to the user experience.
• [x] RMT's test coverage remains at 100%.
• [x] If my changes are non-trivial, I have added a changelog entry to notify users at package/obs/rmt-server.changes.

Other Notes

Please use this space to provide notes or thoughts to the team, such as tips on how to review/demo your changes.

[suse-tests-pass]

Well Done! Your tests are still passing. https://ci.suse.de/job/scc-RMT-integration-tests/351590/console If the given link has expired,you can force a Prophet rerun by just deleting this comment. (Merged 5ea6b329aeb8e5a1750ff6c9c057b84ca329854d into 9f151f09811d94f59cf11460b814f1faa2f193c8)