Without this patch new test fails with:
Exception: Failed to preprocess config '/etc/clickhouse-server/config.xml': Exception: Element <max_thread_pool_size> has value and does not have 'replace' attribute, can't process from_env substitution. Stack trace:
Signed-off-by: Azat Khuzhin <a.khuzhin@semrush.com>
Such nodes in config are useful as a nodes with default values, that can
be overwritten from ZooKeeper/env.
So after this patch the following is valid, and is interpreted as
default value 86400, and can be overwritten via env:
```xml
<asynchronous_metrics_update_period_s replace="1" from_env="CH_ASYNCHRONOUS_METRICS_UPDATE_PERIOD_S">86400</asynchronous_metrics_update_period_s>
```
While the following is not:
```xml
<asynchronous_metrics_update_period_s from_env="CH_ASYNCHRONOUS_METRICS_UPDATE_PERIOD_S">86400</asynchronous_metrics_update_period_s>
```
Signed-off-by: Azat Khuzhin <a.khuzhin@semrush.com>
