In order that something consuming an artifact can have access to extra
builder specific data add the State method which allows the caller to
ask for arbitary values by name.
With go tip, the output from Artifact.String will sometimes be output in a
different order than the tests. Sort the AMI strings before outputting.
See https://travis-ci.org/mitchellh/packer/jobs/28748467 for an example of this
failure.
