setup a cache frontend for the blog

design docs in https://help.torproject.org/tsa/howto/cache/

launch checklist:

1. alternatives listing and comparison (done)
2. deploy a test virtual machine by hand, say cache-01.tpo (done)
3. benchmark the different alternatives (done, ATS and nginx comparable)
4. setup secondary node with Puppet, say cache-02.tpo (done)
5. validation benchmark against both nodes (done)
6. lower DNS to 10 minutes wait an hour (done)
7. open firewall (done)
8. lower DNS to 3 minutes (done, around 2019-11-05 16:00:00)
9. point DNS to caches (done)
10. raise DNS back to 1h if all goes well. (done!)

Post launch tasks:

1. update documentation to fill in information from template (done)
2. hit ratio stats in to Prometheus, separate ticket? (done, although missing syslog buffer, see #32461 (moved))
3. convert existing varnish setups into Nginx (probably requires Puppet refactoring, see #32462 (moved))

Disaster recovery:

1. flip DNS back to backend