certbot(role): support DNS-01 certs using acme-dns

Introduce new configuration structure called certbot__certs, which
allows for different challenge types per cert with the first challenge
type supported being dns-01-acme-dns.

roles/certbot/meta/argument_specs.yaml

@@ -13,6 +13,25 @@ argument_specs:
         type: str
         required: false
         default: 31820
+      certbot__certs:
+        type: list
+        elements: dict
+        required: false
+        default: [ ]
+        options:
+          commonName:
+            type: str
+            required: true
+          # ToDo: subjectAlternativeNames:
+          challengeType:
+            type: str
+            required: true
+            choices:
+              - dns-01-acme-dns
+          dns_01_acme_dns:
+            type: dict
+            required: false
+            # Further checking done in tasks/validate_cert.yaml
       certbot__new_cert_commands:
         type: list
         elements: str