Revert "Disable host key checking in nodepool"

This reverts commit 49f721a0ba3531384bfaf920fdf1f4a606e6fa92.

Reason for revert: This did not work as expected -- all connections received a host key verification error.

Change-Id: Id0be45f2c45e61f513d26bb5a6801e145255d484
diff --git a/nodepool/nodepool.yaml b/nodepool/nodepool.yaml
index b2f3a6e..2f7dfea 100644
--- a/nodepool/nodepool.yaml
+++ b/nodepool/nodepool.yaml
@@ -18,18 +18,6 @@
       - name: main
         max-servers: 4
         use-internal-ip: True
-        # Host key checking is disabled because:
-        # 1) We're using the internal IP so it's slightly less
-        # valuable (fewer attack vectors).
-        # 2) The images we're using appear to have a key baked into
-        # them which is overwritten at boot.  Because we're using the
-        # internal IP, nodepool can end up connecting to the instance
-        # very quickly and retrieving the original host key rather
-        # than the new one (which is likely to be written a couple of
-        # seconds later).  By disabling this in nodepool, we let Zuul
-        # just use the first key it finds (and it's likely to take
-        # long enough that it will have been updated by then).
-        host-key-checking: False
         labels:
           - name: debian-stretch-8G
             instance-type: n1-standard-2