Refactoring, clearer configuration, test-cases

[Argelbargel]

• changes configuration-structure to something (imo) clearer
• allows setting of all options via environment-variables
• use [viper](https://github.com/spf13/viper] for configuration-handling
• used @Boostport's refactorings as base for my own -- split out/renamed storage-code to uploader "classes" -- split vault authentication-code in separate "classes" -- split vault-client/api to make code testable without vault-server
• adds test-cases for those parts of the code i've already understood