Add support for overrides from environment variables

[sungam3r]

One of my colleagues was faced with the problem when trying to set the value of the level override from environment variables. This PR helps to do that. @skomis-mm @nblumhardt I have a suspicion that there is another (simpler) solution and I do not believe that all this time no one has come across this problem. However, fix seems to be obvious and non-breaking affecting only cases when level overrides come from environment variables. Configuration from json files works well - it's safe to use dots there.