Fix shadowing of parameter in scripts

The python registries script used the variable json, which conflicts
with the "import json" module.  This commit changes the name of the
variable to json_dict, which is hopefully a more generic name:

Tested:
Ran script, observed same behavior.  Registries are out of date, but
will be updated in a later commit.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Icedc1d79ff5c9d8bfc8fe5eac96a1c6ea059272a