Add a random sleep time to the rsync cronjob we suggest mirror operators use

Now that we have lots of mirrors, having them all sync’ at the same time is just a recipe to give them slow transfers, and cause scaling issues on the rsync server’s side. Let’s try to insert a 0-30 minutes random delay. This has at least 3 aspects:

• contribute/how/mirror
• the corresponding Puppet class
• the release process doc: in order to avoid blocking on all mirrors to have sync’ed, we’ll want to point (internal) testers to a specific one that has synced already; if it’s not trivial to document how to do that, I’ll add an option to check-mirrors to simply return the first up-to-date mirror it sees while trying them in random order, or something.

Feature Branch: doc/11482-random-mirrors-sync-time

Parent Task: #7161 (closed)

Original created by @intrigeri on 11482 (Redmine)