Update 'execute_on' syntax

 (refs #3461)
I'm not sure I understand how setting values reduces flexibility. At the very most it would just be useless. I do understand that we are not currently making use of the values as they sit in MultiMooseEnum but it is possible to retrieve integer values if a user attempts to do so. It would simply by nice for them to match in case they ever did retrieve a value and attempted to compare it directly against the C enum. The same logic should apply to the Output constants. They should simply all be the same for clarity.