Update 'execute_on' syntax

 (refs #3461)
The output system utilizes a different approach than 'execute_on', setting the values in the MultiMooseEnum it not necessary because all of the additional machinery that exists that converts the MultiMooseEnum to the actual enum values, see Conversion::initExecStoreType and execstore_type_to_enum.

Setting the values here reduces the flexibility, in the current system, the enum values do not matter with for 'execute_on', with exception of the bitwise comparison requirement.