More configuration via environment variables

[antonpirker]

Currently we have a few config options in the SDK that can also be set via environment variables.

To make it easier for our users to deploy their Sentry instrumented project on different environments (development server, staging, production. But also Docker based services, AWS Lambda or other cloud services) it would be good to enable more of our config options to be configurable via environment variables.

This would make our users projects way more portable, and also brings value to all customers that follow the principles of The Twelve-Factor App.

• Come of with a list of config options that make sense to set via env var
• Implement picking those config options from the environment
• Update docs