manage mirror pool through puppet

we currently use multiple separate repositories to manage our mirror pool, which have to be kept in sync with hiera to monitor their speeds. let's manage everything through puppet instead and make sure we can configure the entire pool in 1 place.

To-do

• Add mirror data to hieradata/common.yaml at tails/puppet-code>
• Add Puppet code to generate www.lizard:/srv/tails.boum.org/mirror-pool/mirrors.json from that data
• Add Puppet code to update the website whenever that file changes
• Switch the /srv/tails.boum.org/git/mirror-pool underlay to /srv/tails.boum.org/mirror-pool
• Remove the copy of tails/mirror-pool> at /srv/tails.boum.org/git/mirror-pool
• Figure out the future of the scripts in tails/mirror-pool>
• Archive tails/mirror-pool>
• Remove the copy of mirror-pool.git in Gitolite at puppet-git.lizard
• Update documentation accordingly
• Modify Icinga to remove redundant mirror data from hieradata/common.yaml
• Change Mirrorbits so it's also updated from a Puppet-generated file