letsencrypt: add check for TransIP global_key

* as per comment #3855 (comment)
* fixes #3876

Co-authored-by: Goopher <goopher@live.nl>

letsencrypt/rootfs/etc/services.d/lets-encrypt/run

@@ -271,8 +271,11 @@ elif [ "${CHALLENGE}" == "dns" ] && [ "${DNS_PROVIDER}" == "dns-simply" ]; then
 
 # TransIP
 elif [ "${CHALLENGE}" == "dns" ] && [ "${DNS_PROVIDER}" == "dns-transip" ]; then
-    bashio::config.require.username 'dns.transip_username'
+    bashio::config.require 'dns.transip_username'
     bashio::config.require 'dns.transip_api_key'
+    if bashio::config.has_value 'dns.transip_global_key'; then
+    dns.transip_global_key=$(bashio::config 'dns.transip_global_key')
+    fi
     if (( PROPAGATION_SECONDS < 240 )); then
         bashio::log.info "Increasing DNS propagation limit for TransIP to at least 240 seconds."
         PROPAGATION_SECONDS=240