Allow to provide/override config key values using env variables

The old configurer allowed to override key/values using env variables. This is very suitable when depolying using docker as it is a standard approach there.

I propose we also support this for configurator. The data provided in env should override eventual default configs provided inside bundles. 

If possible we should make sure that the config is still only created once so services do not oscilate.

With some pointers I might  be able to provide a PR for this.