relax DOMAIN regex to allow longer TLDs

Currently a TLD need to be 2 to 5 characters long. I'd recommend removing that restriction.

Concretely there is already one active gTLD that would not fit into the current limit:
`.museum`

Additionally some of the proposed TLDs will be over 5 chars too:
http://en.wikipedia.org/wiki/Proposed_top-level_domain

But what I really need is `.service`, internal DNS we use to resolve to the correct DC.

The other thing to think about is [IDN](http://en.wikipedia.org/wiki/Internationalized_domain_name). I guess they would currently work with punycode but not with UTF-8. However I believe that is a bigger issue that should be addresses separately with lots of tests.