**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.

{
    std::lock_guard lock(shared->clusters_mutex);
    if (!shared->clusters)
    {
        const auto & config = shared->clusters_config ? *shared->clusters_config : getConfigRef();
        shared->clusters = std::make_shared<Clusters>(config, settings);
    }
    return shared->clusters;
}
void Context::setClustersConfig(const ConfigurationPtr & config, const String & config_name)
{
    std::lock_guard lock(shared->clusters_mutex);
    
    if (shared->clusters && isSameConfiguration(*config, *shared->clusters_config, config_name))
        return;
    auto old_clusters_config = shared->clusters_config;
    shared->clusters_config = config;
    if (!shared->clusters)
        shared->clusters = std::make_unique<Clusters>(*shared->clusters_config, settings, config_name);
    else
        shared->clusters->updateClusters(*shared->clusters_config, settings, config_name, old_clusters_config);
}
void Context::setCluster(const String & cluster_name, const std::shared_ptr<Cluster> & cluster)