Refactor: cts: De-nest cts-lab --fencing arg processing

If args.fencing is unset, then its value is None, so args.fencing in
["0", "no"] will be false. Also, DoFencing should be false if and only
if args.fencing in ["0", "no"]. So as far as I can tell this preserves
behavior.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>