Parametrize the json bodies with configuration variables

The JSON payloads in the json folders currently need to be manually edited to be configured to suit the FUT.

They could be parametrized or "templated" so that at run time they are filled with the information from the environment/variables.txt file.

This is particularly relevant when the JSON payloads should include values that are computed at runtime from the test system, e.g. the callbackURL for notification tests: in automated tests system, the actual endpoint may not be known before test execution time, and the JSON payload need to be edited accordingly by the test system.

Proposed solutions

• Use a complete templating system
• Use Python string formatting capabilities