default krb_rdns to True

koji-gc fails to run if you do not set a value in your config.
it should default to a valid value.

Signed-off-by: Dennis Gilmore <dennis@ausil.us>

util/koji-gc

@@ -160,7 +160,7 @@ def get_options():
             ['keytab', None, 'string'],
             ['principal', None, 'string'],
             ['krbservice', None, 'string'],
-            ['krb_rdns', None, 'boolean'],
+            ['krb_rdns', True, 'boolean'],
             ['runas', None, 'string'],
             ['user', None, 'string'],
             ['password', None, 'string'],