Fix issue 1736: add default values for java-implementation-class properties

This change adds default values for all java-implementation-class style properties where appropriate. These properties are usually defined in an abstract definition but have sensible default values in their sub-definitions. For example, the PasswordValidatorConfiguration definition defines a "validator-class" property which is mandatory but has no sensible default value. However, its derived definition DictionaryPasswordValidatorConfiguration does have a sensible default value for this property, DictionaryPasswordValidator, so this change adds a "property-override" to this sub-definition which defining this default value.

Having default values defined for java-class type properties will make it much easier for administrators to create new configurations as they will no longer need to figure out which class is appropriate (they are still free to override the default with their own implementation if they wish).