setup configobj valid ranges for config settings

[binary1230]

discussion from #1

ConfigObj (the library we use for our config files which provides that handy specfile concept) already has a way to indicate a valid range for integers. You can just drop this unit test and instead modify your configspec to read something like:

barcode_salt = integer(default=0, min=0, max=1000000)
barcode_event_id = integer(default=0, min=0, max=255)
barcode_key = string(min=10, max=10)  # min and max refer to length for strings

The length restrictions are strings are from memory, so that might actually have a different syntax. Relatedly, if those config options are required for the plugin to function, you probably don't want default values, since you probably want an exception to be raised if they're not provided.