upgrade all machines to Debian Stretch

catch-all tasks to track the progress of upgrading Software Heritage machines to Debian Stretch

machines:

• banco
• beaubourg
• getty
• grand-palais
• louvre
• moma
• pergamon
• petit-palais
• prado
• saatchi
• somerset
• tate
• uffizi
• worker{01-16}
• worker{01-08}.euwest.azure

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Migrated from T761 (view on Phabricator)