Environment variables are not processed immediately

Environment variables are not processed immediately

Problem
-------

Environment variables are not processed immediately by the Service
Participant.  For many configuration keys, this is irrelevant.  For
others, like those processed in
`Service_Participant::ConfigReaderListener::on_data_available`, it
applies the configuration immediately.

A related problem is that if there are no arguments or configuration
file, then the configuration from environment variables is not
applied.  This prevents an "environment variable only" configuration
from working properly.

Solution
--------

Invoke the listener after each environment variable is processed.