[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
This is the kind of reformatting I mean that falls outside the scope of the change actually made.