passing/setting processing-related environment variables in ocrd_network

[bertsky]

Shouldn't we also allow the user to pass the ocrd_utils.config options to the server, somehow? (In v3, you'll remember, we introduced fine-grained controls like OCRD_MISSING_INPUT, OCRD_MISSING_OUTPUT, OCRD_EXISTING_OUTPUT, that should in principle be configurable on a case-by-case basis. Without any such delegation, one would have to live with the defaults set on the server side, so there's not much use for them in the network scenario...)

Originally posted by @bertsky in #1303 (comment)