Add support for puppetserver

Set server_implementation to 'puppetserver', which will disable Passenger,
install puppetserver and start it as a standalone service.
I think the only reason it's here is to mirror the agent installation.  puppet-common is listed under the agent package list because we allow the user to specify the Puppet version, and so we have to specify both packages else you get ugly dependency errors if downgrading etc.

As we don't let the user specify the server version (a bug in itself I guess), I don't think it's necessary.