Comment 1 for bug 1593030

Root cause is, tacker using a "string" param_values [1] and passes that to ToscaParser parsed_params [2] which expects it to be in json,

params = {'db_name': 'my_wordpress', 'db_user': 'my_db_user',
                  'db_root_pwd': 'mypasswd'}
        tosca = ToscaTemplate(parsed_params=params,
                              yaml_dict_tpl=yaml_dict_tpl)

Options to fix are,

a) convert the string param_values to json object before passing to ToscaTemplate
b) change the API / db to use json object to pass parameters

(b) is the preferred approach.

[1] https://github.com/openstack/tacker/blob/master/tacker/vm/infra_drivers/heat/heat.py#L288
[2] https://github.com/openstack/tacker/blob/master/tacker/vm/infra_drivers/heat/heat.py#L293