add more configurable options via env vars (6.1.x)

[erral]

See #153 for discussion

[jensens]

Instead of blowing this up here I would go for using cookie-cutter-zope-instance which supports all available options already. It comes with a helper transform-from-environment.py to turn environment variables into settings.

[davisagli]

@jensens I like that idea, but we also need to be careful we don't break things if cookiecutter-zope-instance uses different environment variable names from the ones that have been here for some time. Maybe someone can investigate at the Alpine City sprint?

Also note I already merged the same change to the 6.0.x branch

[jensens]

It uses different names, indeed. We could add a compat layer and deprecate the old names for a while. +1 for more at ACS

Requested change has been done.

[mauritsvanrees]

Since the same PR on 6.0.x was already approved and merged, I will do the same here. Afterwards I will make a PR for 6.1.0rc1 which is now ready.