PackageBase: add a list of metadata attributes
PackageBase: add a list of metadata attributes

These attributes are ignored when doing a content hash of a package.

mfherbst/spack,matthiasdiener/spack,EmreAtes/spack,EmreAtes/spack,iulian787/spack,krafczyk/spack,tmerrick1/spack,mfherbst/spack,LLNL/spack,krafczyk/spack,matthiasdiener/spack,matthiasdiener/spack,LLNL/spack,tmerrick1/spack,tmerrick1/spack,iulian787/spack,krafczyk/spack,tmerrick1/spack,mfherbst/spack,EmreAtes/spack,matthiasdiener/spack,LLNL/spack,EmreAtes/spack,tmerrick1/spack,matthiasdiener/spack,mfherbst/spack,iulian787/spack,LLNL/spack,krafczyk/spack,iulian787/spack,mfherbst/spack,EmreAtes/spack,krafczyk/spack,iulian787/spack,LLNL/spack