infrapolicy/cost estimation: provide an auto detect of the tfplan_path file

[gaelL]

Those both features rely on a tfplan file provided by the Terraform concourse resource.

Common error from our users is a wrong path given as parameter.
90% of the time the file have the same name. We could by default when the path if not specified, try to auto detect it.
Like find . -name tfplan.json as default behavior