Install Puppet using puppet-installer

In order to

* reduce copy-and-paste between skeleton and modules
* allow to change installer code in one place (https://github.com/Jimdo/puppet-installer)

This basically mimics Chef's `install.sh` script, see https://docs.getchef.com/install_omnibus.html

Note: Of course, this is for Vagrant development machines only.
I guess `https://raw.githubusercontent.com/Jimdo/puppet-installer/master/puppet-installer.sh` isn't that long anyway.