Resolve domain names recursively when get CNAME records

Hi Dr. Nic,

I found that `bosh cf create` fails when I gave a `x.x.x.xip.io` domain name for the DNS value. I examined the code and realised that the current `resolve_dns` does not support CNAME records in the validation.
This commit fixes the problem by resolving CNAME records recursively.

Thanks.