vars syntax to read json with relative paths

[k2tzumi]

Currently it is loaded relative to the runn execution path, but I would like to load it relative to the scenario file.
We believe that the above specification is preferable when trying to manage multiple scenario files distributed in multiple directories.

[k1LoW]

I agree with the proposal 👍

There is one thing we need to be concerned about.
There may be a case in the future where the Runbook is not a file.

• When YAML is passed as a string instead of a file in Go code
• When YAML is passed from STDIN