Remove ssh host keys during image build

If we're going to remove cloud-init, then we need to make sure we don't
bake in ssh host keys.

Change-Id: Id67934924053cce77b440ddb521112fe1f96e76a

nodepool/elements/puppet/bin/prepare-node

@@ -58,3 +58,5 @@ set -e
 
 # Make sure resolv.conf settings don't break dib
 echo "nameserver $NODEPOOL_STATIC_NAMESERVER"> /etc/resolv.conf
+
+rm -f /etc/ssh/ssh_host_*

nodepool/scripts/prepare_node.sh

@@ -173,5 +173,7 @@ sudo -H virtualenv /usr/zuul-swift-logs-env
 sudo -H /usr/zuul-swift-logs-env/bin/pip install python-magic argparse \
     requests glob2
 
+sudo rm -f /etc/ssh/ssh_host_*
+
 sync
 sleep 5