Log Message:
Update databases/py-barman to 1.6.1.

- Add --peek option to get-wal command to discover existing WAL
  files from the Barman's archive
- Add replication-status command for monitoring the status of any
  streaming replication clients connected to the PostgreSQL
  server. The --target option allows users to limit the request to
  only hot standby servers or WAL streaming clients
- Add the switch-xlog command to request a switch of a WAL file to
  the PostgreSQL server. Through the '--force' it issues a
  CHECKPOINT beforehand
- Add streaming_archiver_name option, which sets a proper
  application_name to pg_receivexlog when streaming_archiver is
  enabled (only for PostgreSQL 9.3 and above)
- Check for _superuser_ privileges with PostgreSQL's standard
  connections (#30)
- Check the WAL archive is never empty
- Check for 'backup_label' on the master when server is down
- Improve barman-wal-restore contrib script
- Treat the "failed backups" check as non-fatal
- Rename '-x' option for get-wal as '-z'
- Add archive_mode=always support for PostgreSQL 9.5 (#32)
- Properly close PostgreSQL connections when necessary
- Fix receive-wal for pg_receive_xlog version 9.2