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