Revert "Revert "Disable host key checking in nodepool""

This reverts commit 1bf66605b7652f0cad5e63cb2294b4fa4a029d37.

Reason for revert: The Zuul executor should be updated to handle this case now.  It has been restarted with https://review.opendev.org/739986 in place.

Change-Id: I401abf5ef55a9fcd455bce21ff56c2590f968e60