Configuration by Environment Variables

*This issue was originally created by [**@genofire**](https://github.com/genofire) at .*

**Problem:**

In a K8s it is really common to run dependencies by other applications like an database operator (zalando's postgres operator). Which generates k8s secrets, for credentials like username and password.

It would be nice to use them directly (instatt of generate configfiles, stored in new k8s secrets with helm or other tooling). Therefore exists the possibility to use envFromSecrets in k8s deployments.

**Description/Solution:**

So please make it possible to give dendrite configuration parameters with env variables.

**Implemented Suggested:**

Maybe you like to use the golang library [viper](https://github.com/spf13/viper) and cobra from spf13 for it.

The issue names no Dendrite files, tests, or configuration entry points. Start by locating how configuration is loaded and compare that with Kubernetes envFromSecrets and the suggested viper/cobra approach; done means the requested configuration parameters can be supplied through environment variables without generated config files.

Written by the indexing model from the issue text.