Review and validate config namespaces

[richtja]

Is your feature request related to a problem? Please describe. After a discussion in https://github.com/avocado-framework/avocado/issues/5257 we define the following concern: Whit TestSuite.from_config(job_config) when using the Job API, currently we are assuming this is a config dictionary with only job.* namespaces. So if someone adds a different namespace here it will override default settings, creating inconsistency.

Describe the solution you'd like We need to better document and validate the namespaces here.