don't bail (due to set -e) if ssh-keyscan fails

We don't need to record the ssh host key in most cases given that we
generally deploy signed ssh host keys, but I suspect we might have the
occasional backup target where that doesn't apply (e.g. clusters?)

Regardless, if we can't scan the host key the right behaviour is for
the script to continue on and set up the backup. If the backup then
fails due to the absent host key, we will be alerted and take suitable
action. Right now, the failure mechanism is that we silently don't finish
setting up the backup, the backup never gets enabled, and we don't
realise we don't have a backup - eek.