Move puppet user creation to install

Defining the user puppet in config causes a dependency cycle if the user is
using git_repo=true:

(Exec[restart_puppet] => Class[Puppet::Server::Passenger] => Class[Puppet::Server::Config] => User[puppet] => File[/etc/puppet/rack/config.ru] => Class[Puppet::Server::Passenger])

By moving it to install we ensure it's already there.