Add validate-json tox target

This is a simple JSON validation script based on demjson python library,
we do some shell script trickeries to exit properly if there is a
failure for tox (should probably patch demjson to properly exit
instead).

Change-Id: I930908336deef7daeaab5b55ba2031c64698d880
Implements: blueprint json-validation-gate
Merge "Add validate-json tox target"