only register letsencrypt account key if it changed

tasks/letsencrypt_setup.yml

@@ -1,4 +1,5 @@
 - name: generate letsencrypt account key
+  register: letsencrypt_account_key
   openssl_privatekey:
     path: /etc/ssl/letsencrypt_account.key
     size: 4096
@@ -7,6 +8,7 @@
     mode: 0600
 
 - name: register letsencrypt account
+  when: letsencrypt_account_key is changed
   acme_account:
     account_key_src: /etc/ssl/letsencrypt_account.key
     state: present