Unbound initialization and validation funcs from global Config

In order to remove dependency on the global config.Config var we need to break dependencies between Config and it's initialization and validation functions. Most of them now represented as a methods on the config.Cfg type. It allows us to setup required Cfg struct for the tests to run without need to sync changes done on the shared global Config var. The tests that were heavily dependent on the Config var now depend on the struct initialized inside the test instead.

Part of: #2699 (closed) Closes: #3166 (closed)