Revert "default krb_rdns to True"
    
    This reverts commit 02128ee5777b560619314cc0220e1ed7ca16b28c.
    
    The second column in cfgmap is an alias and "True" is not a valid alias.
    It was mistaken in the reverted commit as a default value, which it is
    not. Reverting this commit should fix the following traceback that
    koji-gc in Fedora currenty raises:
    
    Traceback (most recent call last):
    File "/usr/sbin/koji-gc", line 901, in <module>
    options, args = get_options()
    File "/usr/sbin/koji-gc", line 159, in get_options
    if config.has_option(*alias):
    TypeError: has_option() argument after * must be a sequence, not bool