MWEB-954 Clean up before switching to deploytools plugin for Wikia Maps

While working on deploytools plugin changes for Wikia Maps we learnt existence of `/etc/environment` file which have most of the environment variables we were setting in our runit script. We've decided to use the variables set in the file. So:
* we changed usage of `WIKIA_PROD_DATACENTER` to `WIKIA_DATACENTER`,
* removed unnecessary logic with lower casing the value of "datacenter variable",
* updated `README.md`,
* and did small refactoring of names -- we went with the naming convention taken from `/etc/environment` and `Settings.yml` files (`prod` instead of `production`, `dev` instead of `devbox`).

Along with these changes we had to change few things in couple of other places:
* https://github.com/Wikia/config/pull/674