Add tf_vars to the data structures sent in terraform push.
This takes any value of type []interface{} or map[string]interface{} and
marshals it as a string representation of the equivalent HCL. This
prevents ambiguity in atlas between a string that looks like a json
structure, and an actual json structure.
For the time being we will need a way to serialize data as HCL, so the
command package has an internal encodeHCL function to do so. We can
remove this if we get complete package for marshaling HCL.
James Bardin