Changed Allow User Registration setting to OFF for new Joomla installati...

...ons

I have changed the default "Allow User Registration" setting from ON to OFF as discussed in this topic: https://groups.google.com/forum/#!topic/joomla-dev-cms/I5QR5pLaIIA

I also added the change in these two test files:
/joomla-cms/tests/unit/stubs/database/jos_extensions.csv
/joomla-cms/tests/unit/suites/libraries/joomla/utilities/stubs/test.xml

The setting should *not* be applied to existing websites and therefore I did not create any SQL update files.