cubicweb #13889793 all-in-one configuration as python code [validation pending]

It would be rather useful to be able to configure cubicweb's all-in-one and source settings through some python code.

This is the case in the django world

One of the immediate advantages would be to benefit from that in the cubicweb docker deploys, instead of having to sed through all-in-one.conf the could try to look in environment variable and default to values.

One example of that use case can be read in the sentry (django app) configuration in their docker setup :

A combination of py / txt configuration file / environment variable could be decided.

A first proof of concept could be a python script that generates an all-in-one.conf from environment variables (which would skip/avoid sed on those files).

done in3.24.0
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patch[doc] Document configuration override by environment variables [applied]Let configuration option be overridden by environment variables [applied]