software/rapid-cdn: avoid RegExp to validate hostnames

Using RegExp to validate hostnames is a bad practice, and has a lot of reasons to be wrong.
On top of that, the JSON Schema specification allows, since draft 7, to validate hostnames
against an IDN hostname, by using the `idn-hostname` format.

With these changes, IDN are now supported (.рф and .中國 for instance), and long TLD
should not be a problem anymore.