Override functionality for reactive config

[fit51]

Basic Idea: one config storage can override the other.
It may be useful if your config storage doesn't support per-host configuration, but you need to override some piece of config locally. Ex. etcd.

Possible scenario

Your configuration is stored in etcd clusters that are unique per qa/prod/other environments.

1. Though, you have some static configuration ex: hostname, unique-id, etc that apply to each app node independently...You need an ability to specify per-node configuration, and update it.
2. Sometimes you have to override some general configuration to run tests on part of an application traffic. So, you override config on specific nodes locally.