Configure Passenger in separate .conf file on Debian so PassengerRoot isn't lost

The changes in 005bf615c are also necessary for Ubuntu 14.04 which now
ships Passenger 4.x

I've tested this with Debian wheezy too, to check it doesn't break older Passenger versions - in general I think it should be fine to default to what's shipped with the OS.

Also refs https://github.com/puppetlabs/puppetlabs-apache/pull/698