In an effort to make the EC2 Datasource's search under ec2.archive.ubuntu.com
resilient against dns redirection, we add some code to is_resolvable.
One future enhancement for this would be to protect against server side
round robin results. Ie, if 'bogus-entry' returned 10.0.1.1 one time, and then
10.0.1.2 a second time. We could check if results where within the same 3
octets, and assume invalid if they were.