feature/immutable-env

This PR is the ongoing development for new env handling described in this [draft](https://github.com/soundcloud/visor/blob/feature/immutable-env/doc/immutable-env-design.md). 

### tl;dr
* control on instance level
* immutable
* configuration creation api
* atomic configuration changes
* reasoning about runtime behaviour 

* * *
*The commits in this branch will be cleaned up in time and likely stashed to a single coherent change-set*