**Describe the issue**
ClickHouse is unable to start up after upgrading from 20.8 to 21.8. 

There is a replicated table with `:` in zk path and it couldn't be loaded with:

`2021.09.15 03:49:58.435071 [ 633075 ] {} <Error> Application: DB::Exception: Unknown auxiliary ZooKeeper name '/test_prefix'. If it's required it can be added to the section <auxiliary_zookeepers> in config.xml: Cannot attach table `db1`.`table1` from metadata file /var/lib/clickhouse/metadata/db1/table1.sql from query ATTACH TABLE db1.table1 (`field1` String, `date` Date) ENGINE = ReplicatedMergeTree('/test_prefix:db1/{shard}/table1', '{replica}') PARTITION BY toMonth(date) PRIMARY KEY field1 ORDER BY field1 SETTINGS index_granularity = 8192: while loading database `db1` from path /var/lib/clickhouse/metadata/db1
`

**Expected behavior**
ClickHouse server is able to startup after upgrade without schema change. "test_prefix" in "/test_prefix:db1/{shard}/table1" is not interpreted as an auxiliary ZooKeeper name.

{
    return getClusters()->getCluster(cluster_name);
}
void Context::reloadClusterConfig() const
{
    while (true)
    {
        ConfigurationPtr cluster_config;
        {
            std::lock_guard lock(shared->clusters_mutex);
            cluster_config = shared->clusters_config;
        }
        const auto & config = cluster_config ? *cluster_config : getConfigRef();
        auto new_clusters = std::make_shared<Clusters>(config, settings);
        {
            std::lock_guard lock(shared->clusters_mutex);
            if (shared->clusters_config.get() == cluster_config.get())
            {
                shared->clusters = std::move(new_clusters);
                return;
            }
            
        }
    }
}
std::shared_ptr<Clusters> Context::getClusters() const