plugin-caller: add _readdomainconf stub

it's used by the dns_hetzner (and dns_nsd) plugins.

reported in our community-forum (and tested there as well):
https://forum.proxmox.com/threads/.105879/

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>

src/proxmox-acme

@@ -696,6 +696,11 @@ _readaccountconf_mutable() {
   _readaccountconf "$1"
 }
 
+# key
+_readdomainconf() {
+  echo "${!1}"
+}
+
 # no-ops:
 _clearaccountconf() {
   return 0