Move Chrome's dmg_package source, checksum & dmg_name to attributes file

Notes:

      To use a custom version, put something like this in your
     soloistrc:

        node_attributes:
         sprout:
           chrome:
             dmg:
               source: https://dl.google.com/chrome/mac/stable/GGRO/googlechrome.dmg
               checksum: 6f4673c02ec38d4d8a480dc1e41caa97f49320ed9898bcbe1abde3272b636061
               dmg_name: googlechrome