Validate job runner
We should be able to create an invalid job runners (e.g. for tests but most importantly because we need to do it to kill zombie jobs). Therefore I moved the check_config_exists() validation check to a dedicated `.validate()` method that is called only when the job will be run.

This should fix the kill_zombies failures that make all our workers crash in loop now.
Validate job runner: We should be able to create an invalid job runners (e.g. for tests but most importantly because we need to do it to kill zombie jobs). Therefore I moved the check_config_exists() validation check to a dedicated `.validate()` method that is called only when the job will be run.

This should fix the kill_zombies failures that make all our workers crash in loop now.