Add no-agents option to omd backup

[gradecke]

General information

omd backup -N is a great way to get a small backup with only the config.
That is until one starts to use the bakery, the bakery can easily add gigabytes of data to the backup.
This PR introduces a feature to also exlude the baked agents

Proposed changes

• What is the expected behavior?
  Default behaviour of "omd backup" is unchanged
  however, when running with -N to get a small backup, var/check_mk/agents/* is excluded
  omd backup -h -> for help is also changed to reflect the changed behaviour

I think changing (or rather "enhancing") the default behaviour of -N makes sense, as the agents can easily be rebuilt and doing any backup with -N probably already means you really only want a config backup :)

[gradecke]

please note, between the last 3 commits, there was no change in actual code or formatting, just within the string, not because I'm particularly into grammar or anything, but I always needed to retrigger the unit tests and once mypy tests that failed for reasons unrelated to my changes.