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.

software/rapid-cdn/instance-input-schema.json

 {
-  "$schema": "http://json-schema.org/draft-04/schema",
+  "$schema": "http://json-schema.org/draft-07/schema",
   "properties": {
     "-frontend-quantity": {
       "description": "Quantity of Frontends Replicate.",
@@ -20,7 +20,7 @@
     },
     "domain": {
       "description": "Domain used to generate automatic hostnames for slaves. For example 'example.com' will result with slave hostname 'slaveref.example.com'.",
-      "pattern": "^([a-zA-Z0-9]([a-zA-Z0-9\\-]{0,61}[a-zA-Z0-9])?\\.)+[a-zA-Z]{2,6}$",
+      "format": "idn-hostname",
       "title": "Domain",
       "type": "string"
     },

software/rapid-cdn/instance-slave-input-schema.json

 {
-  "$schema": "http://json-schema.org/draft-04/schema",
+  "$schema": "http://json-schema.org/draft-07/schema",
   "properties": {
     "custom_domain": {
       "description": "Custom Domain to use for the website. Shall contain only letters, numbers and -, and can look like example.com, first2.example.com special-site.example.com.",
-      "pattern": "^([a-zA-Z0-9]([a-zA-Z0-9\\-]{0,61}[a-zA-Z0-9])?\\.)+[a-zA-Z]{2,6}$",
+      "format": "idn-hostname",
       "title": "Custom Domain",
       "type": "string"
     },