Allow configuration of appdata directory location

Allow configuration of appdata directory location: I'm using Nextcloud 14.0.4 via the docker image. On my host machine, I mount a CIFS share to my filesystem, and I use this mounted directory as the data directory for Nextcloud. For performance reasons, my expectation was that Nextcloud would use the data directory as little as possible to avoid IO contention. Basically, it should only ever be used when an actual backed up file is accessed (actual files they sync). After all, the only reason I mount this way is so that the main storage for users goes to my NAS.