Add variables substitution capabilities
We keep our deployment configurations in octopus variables and use json substitution the replace them on deploy time
This could be improved in a few ways:
- Use the json variables substitution feature on other step templates than deploy a package (for example: run a script)
- have to ability to substitute also yml files
- give a variable a null value, the only option now is tho give him empty string, which influence the code.
This would be particulairly useful for substituting environment specific JSON inside of YAML files.