Sort AMI names in Artifact.String - fixes random test failures

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.
Merge pull request #1305 from lflux/sort-ami-strings

Sort AMI names in Artifact.String - fixes random test failures