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
I would true as default and make sure on setup false is written to the config.

this way the file remains readonly in case it is empty (known race condition).
an admin could then after setup manually set the config to readonly if necessary/required