Fix metaclass __new__() parameter naming: cls ==> mcs

************* Module salt.utils.parsers
C0204: 43,4:MixInMeta.__new__: Metaclass class method __new__ should have 'mcs' as first argument
C0204: 54,4:OptionParserMeta.__new__: Metaclass class method __new__ should have 'mcs' as first argument
Merge pull request #6014 from hulu/mcs

Fix metaclass __new__() parameter naming: cls ==> mcs