roles/nginx: minor rework of acme.sh tasks

After the inital acme.sh script is downloaded and bootstrapped we
can remove it. If a host already has been bootstrapped then there
is no need to download it and do it over again.

roles/nginx/defaults/main.yml

@@ -33,8 +33,8 @@ letsencrypt_root: /etc/ssl
 
 # Location where to save initial acme.sh script. After installation the script
 # will automatically create its home in the /root/.acme.sh directory (including
-# a copy of the script itself).
-letsencrypt_acme_script: /root/acme.sh
+# a copy of the script itself). The initial script is not needed after.
+letsencrypt_acme_script_temp: /root/acme.sh
 letsencrypt_acme_home: /root/.acme.sh
 
 # stable is 1.20.x