Allow to globally configure environ_prefix

[husio]

In the current implementation every declared value is mapped to a shell variable with a DJANGO_ prefix by default. Prefix value can be overwritten and it must be done per variable declaration.

It would be amazing to be able to declare the default prefix value globally for the whole configuration instead of specifying the same environment prefix for every variable declaration. I think this could be implemented in many ways, ensuring backward compatibility. For example by providing a decorator for the configuration class:

@use_environ_prefix(None)
class Base(Configuration):
    # If no environ_prefix is provided, default is used.
    # No prefix. Use FOOBAR_KEY environment valiable.
    FOOBAR_KEY = values.SecretValue()

    # Exiplicit declaration of the environ_prefix for the value always wins.
    # Use MYAPP_DEBUG environment variable.
    DEBUG = values.BooleanValue(False, environ_prefix="MYAPP")

What do you think?

[jklingen92]

This would be immensely helpful, especially because many many environment settings are meant to configure installed apps and have predefined names. It's a big hassle to have to include the environ_prefix kwarg on every single environment variable.

[EnriqueSoria]

I think that allowing to change default prefix is a must. Meanwhile you can use this hack:

# Override default value of environ_prefix
Value.__init__ = partialmethod(Value.__init__, environ_prefix=None)

[robhudson]

This would help ease the transition to django-configurations since I would wager many projects don't prefix all their Django specific env vars with "DJANGO_", although I agree that's a nice and useful pattern.

I wrongly assumed that by setting the environ_name this is the env var used for look up without any prefix. I was surprised when most of my settings weren't being set correctly. Once I realized it was because of the prefix, I lazily didn't want to go adding the prefixes on every single call. So I ended up here looking to see if I was the only one.

The above partialmethod hack worked for me, even for the subclasses. Thanks!

[g-nie]

I gave this a go: #400
Feel free to review the PR 📝