(PUP-2691) Re-adding the missing ‘description’ property to P::MT::Metadata.

Prior to Puppet 3.6.0, users had a ‘description’ property which they
could reliably expand into their module skeleton’s ERB templates. This
property was removed from the default set of properties in Puppet 3.6.0,
which effectively broke template expansion for users relying on it.

This change ensures that users can continue to rely on this property.