Add variable expension/extrapolation

[vduseev]

Summary

Implement template like functionality using Jinja.

Given:

{
  "a": "my",
  "b": "{{ a }}var",
  "{{ a }}-{{ b }}": ["{{ b }}"]
}

Should evaluate to:

{
  "a": "my",
  "b": "myvar",
  "my-myvar": ["myvar"]
}