hotfix created empty .tar.gz in 2.2.7+

[Slach]

system.disks
default, disk2

disk_mapping:
   "default": "/var/lib/clickhouse/"
   "default-1-1": "/var/lib/clickhouse"
   "disk2": "/var/lib/clickhouse2/"
   "disk2-1-1": "/var/lib/clickhouse2"

wrong empty .tar.gz is created, which can't be restored

[Slach]

A root reason for wrong create parts in 2.2.7,
2.2.6 is not affected
v2.2.5...v2.2.7#diff-2da9a9de21c1940972736f4bfc8c28eda6344c5198c018ff300bd04c4baeaf90R529-R533

look screenshot

0. code which mixed system.disks and disk_mapping instead of just replacing disk path, was added @AlexAkulov 😉
   it is helpful, when you make to download backup with different disk structure with current system.disks

1. changes in 2.2.7, relates to Use UNFREEZE TABLE in ClickHouse after backup finished to allow s3 disks unlock and delete remote keys during merge #423
   we make UNFREEZE and don't delete /var/lib/clickhouse/data/db/table/shadow/part_name after moving hard link files to /var/lib/clickhouse/backup/backup_name/shadow/db/table/disk_name/part_name/
   we make UNFREEZE after all disks processed for prepare properly delete S3 remote keys because UNFREEZE makes decrement link count fields in local metadata files in /var/lib/clickhouse/disks/s3_disk/data/db/table/part_name/*.*

2. so, a wrong disk got a non-empty part list
   and this non-empty part list added to backup_name/metadata/db/table.json
   JSON serialization of the empty list, just write nothing to table.json under a wrong disk name in 2.2.6-

3. empty directories with part_name in /var/lib/clickhouse/backup/backup_name/shadow/db/table/wrong-disk-name during uploading create empty archive files because we just iterate directories for creating a list of files for uploading

4. because *.tag.gz files empty, so the download process failed because part_name directory was not created after downloading