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.
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