The app_key config entry for zuul actually wants a path to a file, not
the key content itself. Write it to disk and update the config.
Do the file writing in site.pp and not puppet-zuul because it's an
arbitrary filename/content. A zuul user could have zero or many github
connections, and the connection data is stored in a hash in hiera, so
there's not a super great way to add key writing support to puppet-zuul
itself at the moment. It's also a single file.
Change-Id: I43f93f59b9a82186a60734810a277edeac67bbac
