add metadata.json (before new release)

metadata.json is now required on the forge.
further, the forge recommends we add compatibility information as far as
possible. this file is essentially copied from puppetlabs-apache.
add ALL the Operating Systems
Protect the second reference to $package_name in manifests/init.pp, so
that the unbound package can be installed externally to the unbound
module by setting $package_name to false.