Fix KDC race in t_unlockiter.py

The second KDC startup in t_unlockiter.py could race with the
garbage-collected shutdown of the first, causing the second one to
fail to bind the listening port.  Avoid the situation by setting
start_kdc=False, because there doesn't need to be a KDC running for
these tests anyway.  Also use create_user=False and create_host=False,
because those principals aren't necessary either.

ticket: 7977