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/README.md

@@ -8,7 +8,15 @@ Also see the following documentation for a full How-to on how to get certificate
 
 ## Required Arguments
 
-For the required arguments look at the [`argument_specs.yaml`](./meta/argument_specs.yaml).
+- `certbot__acme_account_email_address`: The E-Mail address to use for the ACME account.
+
+## Optional Arguments
+
+- `certbot__certificate_domains`: The domains for which to obtain a certificate using the HTTP-01 challenge.
+- `certbot__http_01_port`: The port number the bot listens on. Should be `80` if directly exposed to the internet.  
+   Defaults to `31820` (for the public-reverse-proxy setup).
+- `certbot__new_cert_commands`: A list of commands to execute after getting a new certificate. Will be added into a bash script.  
+  Defaults to the empty list (`[ ]`).
 
 ## `hosts`
 

roles/certbot/defaults/main.yaml

@@ -1,2 +1,3 @@
+certbot__certificate_domains: [ ]
 certbot__http_01_port: 31820
 certbot__new_cert_commands: [ ]

roles/certbot/meta/argument_specs.yaml

@@ -2,25 +2,18 @@ argument_specs:
   main:
     options:
       certbot__acme_account_email_address:
-        description: The E-Mail address to give to certbot for the ACME account.
         type: str
         required: true
       certbot__certificate_domains:
-        description: The domains for which to obtain a certificate.
-        type: list
-        elements: str
-        required: true
-      certbot__http_01_port:
-        description: |
-          The port number the bot listens on. Must be 80 if directly exposed to the internet.
-          Default is 31820 for the public-reverse-proxy setup.
-        type: str
-        required: false
-        default: 31820
-      certbot__new_cert_commands:
-        description: >-
-          A list of commands to execute after getting a new certificate.
-          Will be added into a bash script.
+        type: list
+        elements: str
+        required: false
+        default: [ ]
+      certbot__http_01_port:
+        type: str
+        required: false
+        default: 31820
+      certbot__new_cert_commands:
         type: list
         elements: str
         required: false