The buster-backports suite on Debian mirrors is not signed by the
old jessie signing key we have set to verify in reprepro, but also
we're not mirroring Debian 8/jessie any longer anyway. Replace that
list with the 9/stretch and 10/buster signing keys and switch to
longer key IDs which match the names used for them in the Puppet
manifest. Also add Puppetry and keyfile for the buster keys so that
they will be installed accordingly. The official list of keys can be
found here: https://ftp-master.debian.org/keys.html
Change-Id: Ia193f040b2b707329948955eb091a186eabf8096
a22df8264f