preseed: always enable puppet agent explicitly

In newer non-puppetlabs packages coming with Debian and Ubuntu the
puppet agent is disabled by default.

Also, /etc/default/puppet is not delivered with the packages anymore, so
check for it's existence before trying to edit it.
Renaming templates according to Foreman

See here:

https://github.com/theforeman/foreman/blob/develop/db/seeds.d/07-config_templates.rb
Add in initial generic cloudinit userdata template