SSH: do not check IPs

Since we use the cluster name for the SSH known_hosts file, ssh will
always detect a changed IP (since we never connect to the cluster master
name, but the node names), and will complain about it/try to update the
user known hosts file (since that is /dev/null, it doesn't matter, but
it's not nice). So we disable the IP check.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>