knotty.py: fix unknown event bb.event.DiskFull

There is an error:

ERROR: No new tasks can be excuted since the disk space monitor action
is "STOPTASKS"!
ERROR: Unknown event: <bb.event.DiskFull object at 0x2ab6310>

This is because we don't handle the event bb.event.DiskFull in
knotty.py, we don't want the knotty to do anything here since we have
done everything in monitordisk.py, so just ignore this event would fix
the problem.

[YOCTO #3523]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>