Allow read-only configuration

Workaround required for IIS setups running ownCloud to prevent dataloss.

Long-term solution would be to move some configuration settings to the database as described at https://github.com/owncloud/core/issues/11843
> this way the file remains readonly in case it is empty (known race condition).

That race-condition is only triggered on IIS when writing values to the configs (when writing and reading at the same time). This will not be possible anymore with that patch.