**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->storage_policies_mutex);
    if (policy_name.empty())
    {
        String path_with_separator = path;
        if (!path_with_separator.ends_with('/'))
            path_with_separator += '/';
        auto disk = std::make_shared<DiskLocal>("_backups_default", path_with_separator, 0);
        shared->backups_volume = std::make_shared<SingleDiskVolume>("_backups_default", disk, 0);
    }
    else
    {
        StoragePolicyPtr policy = getStoragePolicySelector(lock)->get(policy_name);
        if (policy->getVolumes().size() != 1)
             throw Exception("Policy " + policy_name + " is used for backups, such policy should have exactly one volume",
                             ErrorCodes::NO_ELEMENTS_IN_CONFIG);
        shared->backups_volume = policy->getVolume(0);
    }
    BackupFactory::instance().setBackupsVolume(shared->backups_volume);
}
VolumePtr Context::getBackupsVolume() const