Revisit how the model validation happens

Currently the ValidateModelStepHandler is triggered by AbstractAddStepHandler. Not all add handlers extend AbstractAddStepHandler. Also, it does not seem to be triggered for writes.

Also if used in a composite along the lines of

[standalone@localhost:9990 /] batch
[standalone@localhost:9990 / #] /system-property=xxx:add(value=test)
[standalone@localhost:9990 / #] /system-property=xxx:remove
[standalone@localhost:9990 / #] run-batch 

the handler will get triggered at the end for the add. It will fail for the add since there is no such resource. While that is easy to work around by checking for a NoSuchResourceException around the OC.readResource() it seems that what triggers the use of the ValidateModelStepHandler should be more an intrinsic part of the controller which knows what has and has not been added/removed/written to.