[WFLY-3269] : XML parsing mandating the 'force' attribute on username-to-dn even though it has a default value.

Making force attribute optional with default value 'false' as it is defined in the XSD.

Jira: https://issues.jboss.org/browse/WFLY-3269
Also for testing would be easier to just modify the example configuration here: -
  https://github.com/darranl/wildfly/tree/master/core-model-test/tests/src/test/resources/org/jboss/as/core/model/test/security

Having XML embedded within a test case can cause problems, especially when someone accidentally formats the file - in this case luckily we already have tests in this area.