automation: support and use Debian Buster by default While Debian Buster (Debian 10) was released in July, the AWS AMIs were not published until mid September. This commit teaches the automation system to create AMIs for Debian Buster. Since Debian Buster is the new stable Debian release, we make it the default distribution for automation. Differential Revision: https://phab.mercurial-scm.org/D6917