cubicweb #2096328 Add ability to provide per-instance environment variables as config option [deprecated]

In a similar way debian init scripts can have envvars set up in /etc/default/ files.

The use case I have in mind is vcreview, for which I'd like to set a HGRC environment variable so I can set up some specific mercurial configuration easily per instance.

This is an acceptable way to deal with the problem of configuring https access auth parameters, IMHO.

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