Change TTL of generated records to 10 minutes

Usually an IP for a node will not change, so a long TTL is desired. The
exception is if we assign an elastic IP to a node. In these cases, however
rare, we would like the change to propagate sooner than 24 hours.
Support updating initially empty R53 zones

When updating a completely empty zone, the record list will
be empty (not even a hash - simply nil). The code couldn't handle
that.
Move conditional into relevant subexpression.