Fix logging; raise when expecting one key and finding multiple

[vchudnov-g]

Raise ItemNotUniqueError from get_one() when >1 item matches

• Includes test

Fix logging; log manifests and testplans

• Fixed the logging level for sample-tester binary
• Set a few logging statements to be debug rather than info
• We now report the manifest and testplan files read in via logging.info()

(These changes needed to happen eventually and they were helpful in debugging an issue.)