web: Added LETSENCRYPT_ACME_SERVER environment variable

You can set it when default zerossl doesnt work for you.
For example your server located in the country blocked by ZeroSSL.

env.example

@@ -69,6 +69,8 @@ TZ=UTC
 # Use the staging server (for avoiding rate limits while testing)
 #LETSENCRYPT_USE_STAGING=1
 
+# Set ACME server. Default is zerossl, you can peek one at https://github.com/acmesh-official/acme.sh/wiki/Server
+#LETSENCRYPT_ACME_SERVER="letsencrypt"
 
 #
 # Etherpad integration (for document sharing)