Environment variable for the data path possibly inconsitent

## Describe the bug

Just discussed with @micbar the env variable defining the location of the ocis data folder:
* We describe in our admin docs the env variable `OCIS_BASE_DATA_PATH` in [Define the Infinite Scale Data Path](https://doc.owncloud.com/ocis/next/deployment/general/general-info.html#define-the-infinite-scale-data-path). This is the only variable known to me so far defining the data path depending on the deployment. I got the infos from @wkloucek and I remember that he wrote code to define the locations depending on the deployment.
* But is seems, that there is an additional env variable `STORAGE_USERS_OCIS_ROOT` at [Storage-Users Service Configuration](https://doc.owncloud.com/ocis/next/deployment/services/storage-users.html) doing the same thing.

To be clarified:
* Do these env's do the same thing
* If yes, the `OCIS_BASE_DATA_PATH` variable must be added to the storage_users service
* Where do these env's differ - if they do
Hmmm, usually global overwrites service, but does this apply in this case to the data path? How can it differ for what use case?
* Needs checking if both env's are properly used in the code for the same purpose
* The current description of `STORAGE_USERS_OCIS_ROOT` is not sufficient and needs improvement