Expose a number of additional configuration options through the environment variables

[jonasbardino]

The features exposed in https://github.com/ucphhpc/migrid-sync/issues/42 when done should be exposed here in docker-migrid, too.

This means matching variable additions and modifications of the generateconfs.py call in Dockerfile.*. All variables need to be documented in the doc/source/sections/configuration/variables.rst.