Add test for Zappa.get_domain_name
Add test for Zappa.get_domain_name

This covers the two happy paths.

I am instantiating a Zappa object and things are sufficiently mocked that that
doesn't have any side effects.

I haven't looked at all at the boto route53 docs, I just mocked the return
enough so that the return in get_domain_name would be reached.

Miserlou/Zappa,scoates/Zappa,Miserlou/Zappa,pjz/Zappa,pjz/Zappa,scoates/Zappa