Description of problem:
If a non-json value is used in a json type property, heat only says a non-json value is being used but doesn't indicate which one is wrong :
$ sh overcloud.sh --debug
Running "openstack overcloud deploy" command
Removing the current plan files
Uploading new plan files
Temporary Swift GET/PUT URL parameters have successfully been updated.
Plan updated.
Processing templates in the directory /tmp/tripleoclient-8jpt8_jh/tripleo-heat-templates
Action tripleo.parameters.update execution failed: Error validating environment for plan viper: ERROR: Internal Error
Traceback (most recent call last):

File "/usr/lib/python3.6/site-packages/heat/engine/parameters.py", line 455, in parse
return jsonutils.loads(val)

File "/usr/lib/python3.6/site-packages/oslo_serialization/jsonutils.py", line 264, in loads
return json.loads(encodeutils.safe_decode(s, encoding), **kwargs)

File "/usr/lib64/python3.6/json/init.py", line 354, in loads
return _default_decoder.decode(s)

File "/usr/lib64/python3.6/json/decoder.py", line 342, in decode
raise JSONDecodeError("Extra data", s, end)

json.decoder.JSONDecodeError: Extra data: line 1 column 7 (char 6)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):

File "/usr/lib/python3.6/site-packages/oslo_messaging/rpc/server.py", line 165, in _process_incoming
res = self.dispatcher.dispatch(message)

File "/usr/lib/python3.6/site-packages/oslo_messaging/rpc/dispatcher.py", line 274, in dispatch
return self._do_dispatch(endpoint, method, ctxt, args)

File "/usr/lib/python3.6/site-packages/oslo_messaging/rpc/dispatcher.py", line 194, in _do_dispatch
result = func(ctxt, **new_args)

File "/usr/lib/python3.6/site-packages/osprofiler/profiler.py", line 160, in wrapper
result = f(*args, **kwargs)

File "/usr/lib/python3.6/site-packages/heat/common/context.py", line 423, in wrapped
return func(self, ctx, *args, **kwargs)

File "/usr/lib/python3.6/site-packages/heat/engine/service.py", line 1292, in validate_template
strict_validate=False)

File "/usr/lib/python3.6/site-packages/heat/engine/stack.py", line 220, in init
parent_info)

File "/usr/lib/python3.6/site-packages/heat/engine/stk_defn.py", line 43, in init
template.env.param_defaults)

File "/usr/lib/python3.6/site-packages/heat/engine/hot/template.py", line 222, in parameters
param_defaults=param_defaults)

File "/usr/lib/python3.6/site-packages/heat/engine/parameters.py", line 527, in init
self.params[pd_name].set_default(param_default)

File "/usr/lib/python3.6/site-packages/heat/engine/parameters.py", line 383, in set_default
self._update_parsed()

File "/usr/lib/python3.6/site-packages/heat/engine/parameters.py", line 390, in _update_parsed
self.parsed = self.parse(self.default())

File "/usr/lib/python3.6/site-packages/heat/engine/parameters.py", line 458, in parse
raise ValueError(message)

ValueError: Value must be valid JSON: Extra data: line 1 column 7 (char 6)

Version-Release number of selected component (if applicable):


How reproducible:
Always

Steps to Reproduce:
1. Write a non-json value in a json-type parameter
2.
3.

Actual results:
heat-engine doesn't like it very much

Expected results:
it should not like it but at least give us an indication of where to look at

Additional info: