adding defined type to create a checkcommand object from a source file

Defined type that references a source file for its content as these objects can widely vary between each other.
Would you be able to remove the `checkcommand_` from the front of the `target_dir` and `target_file` params, change the owner and group to `root` and the mode to `0644`?

I can see why it would be good to be 640 and owned by icinga/icinga, but I'll have to revisit that with all of the defined types in the module at some point in the future to make sure things are consistent.