fix(cros_sdk): Use new SDK tarball location!

Note that this is using https://commondatastorage.googleapis.com since
our own name http://storage.core-os.net doesn't resolve yet. Also the
cros_sdk --bootstrap option will just fetch the same tarball since the
bootstrap process is now done from scratch via catalyst instead of from
a Gentoo stage3 tarball. At some point there will be a lot of old code
to clean up.