Added .ocdata file to check for data folder validity

In environments where the data folder is mount from another partition,
it is important to check that the data folder we see is actually the
real one. If the mount failed for some reasons, this fix will make
ownCloud temporarily unavailable instead of causing unpredictable
behavior.

Backport of 3c46dcd from master
Return 503 when a config/data dir error exists

Backport of b619ff6 from master
Increase version to trigger upgrade related to .ocdata
Merge pull request #7819 from owncloud/stable6-datafolderexistence

[stable6] [backport] Added .ocdata file to check for data folder validity