[LE] use progresive delay between validation retries

bin/v-add-letsencrypt-domain

@@ -327,7 +327,7 @@ for auth in $authz; do
             fi
             check_result $E_CONNECT "Let's Encrypt domain validation timeout"
         fi
-        sleep 2
+        sleep $((i*2))
     done
     if [ "$validation" = 'invalid' ]; then
         # Delete DNS CAA record