(maint) Yumrepo boolean values are case insensitive.

When handling boolean values for yum repos, a given value is downcased
before it is checked:

    # yum/config.py
    def parse(self, s):
        """Parse a string containing a boolean value.  1, yes, and
        true will evaluate to True; and 0, no, and false will evaluate
        to False.  Case is ignored.

        :param s: the string containing the boolean value
        :return: the boolean value contained in *s*
        :raises: :class:`ValueError` if there is an error in parsing
           the boolean value
        """
        s = s.lower()
        if s in ('0', 'no', 'false'):
            return False
        elif s in ('1', 'yes', 'true'):
            return True
        else:
            raise ValueError('invalid boolean value')

When validating these parameters we should perform a case insensitive
match to match the yum behavior.