In order to work, it is necessary:
* Write a JSON Patch to file "qa/qa-patch.json" in a source repository.
  (And be careful not to commit it!) The file must conform to RFC6902
  and describes what changes should be done to the QA JSON
  configuration. This is simple as
   [
     { "op": "add",
       "path": "default",
       "value": false },
     { "op": "add",
       "path": "/tests/instance-add-file",
       "value": true }
   ]

* Install "jsonpatch" and "jsonpointer" modules on the machines QA is
  running on. Either using "easy_install" or as Debian packages using
  "pypi-install".

QA only loads the modules if the patch file is present and non-empty.
Therefore no changes to buildbots are needed, if the feature is not
used.

It's advisable to add a git pre-commit hook for the main repository to
prevent changes to "qa/patch.json", keeping it to "[]". Suggestion for
such a hook:

  # Exit code 1 if the patch file isn't just "[]":
  python -c 'import json, sys; \
    exit(0 if json.load(open("qa/qa-patch.json", "r")) == [] else 1)'

Locally it's possible to tell git not to track changes to the file at
all (see https://help.github.com/articles/ignoring-files

):

  git update-index --assume-unchanged qa/qa-patch.json

Signed-off-by: Petr Pudlak <pudlak@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>