reorganisation of constants and validation of config settings

[trz42]

This PR turned out to be larger than originally intended (just validating that required config settings are defined when bot components start). However, it reorganises / brings consistent structure to how and where string constants are defined. Hopefully, this makes the code easier to read.

• string constants being used have been defined in the correct modules
  • when they are used they have to be prefixed with the module, hence it becomes more clear to which module they belong
• a new file tools/cvmfs_repository.py for constants used for the settings of a CernVM-FS repository is added
• both the event handler and job manager verify at the start that required/recommended settings are defined in 'app.cfg'

TODO

• test that it actually works
  • various tests ran in DON'T MERGE {2023.06}[foss/2023b] ~wget v1.21.4~ -> py-cpuinfo v9.0.0 trz42/software-layer#74
  • added issue add test which checks that all strings are defined as constants + verify if all constants are still in use #267 for future improvements
• fix potential hound issues

[Neves-P]

Note: I have not run the bot instance on our site with this PR yet, I have only reviewed the code.

I think it's a good change and it adds needed structure for the bot configuration which will make it easier to setup on other sites too! I've just noted a few nitpicking things which could just be me not understanding the logic, go ahead and reject if that's the case.

[Neves-P]

Looks great to me!