Update puppet roles for wikitech to provision PHP 7.2 and switch from HHVM to PHP 7.2 as the runtime for the site.
In T195392 we are migrating all mediawiki maintenance crons to PHP 7.2.
One of them is cron { 'tor_exit_node_update': which also runs on labweb1001/1002. Trying to convert it to PHP7.2 lead to failure and cron spam because labweb hosts don't have PHP 7.2.
They will need to be upgraded. This means Wikitech needs to move to it.