certbot(role): allow empty list of certificate domains

Also explicitly document that they are used with the HTTP-01 challenge.
This is in preparation for adding a new option with DNS-01 challenge
support.

roles/certbot/meta/argument_specs.yaml

@@ -7,7 +7,8 @@ argument_specs:
       certbot__certificate_domains:
         type: list
         elements: str
-        required: true
+        required: false
+        default: [ ]
       certbot__http_01_port:
         type: str
         required: false