Add default compression type for backup to configuration

currently, only the --compression flag can specify which type to use for compression.

under the ticket add a field to config for the default compression type for the backup command when the flag is omitted.

Q: use the field for oplog compression if it is not specified in the configuration.
A (from sergey.pronin): backup and oplog chunks compression settings should be separated. pitr should not use default compression settings from backup and stays "s2"