Add support for Jsonnet TLAs and clean up cfg tests.

[brettviren]

This requires Jsonnet 0.14.0 which is not yet available in UPS-land.

Jsonnet C++ library has support for a feature called "Top-Level
Arguments" (tla). This PR exposes this support through WCT.

TLA requires a Jsonnet file that results in just a single function. Eg:

$ cat a.jsonnet

function(name, port) {
    name: name,
    port: port
}

Ignoring TLA for a moment, we can import this file into some other
Jsonnet file as normal so that we might call this function with
hard-coded "name" and "port" values.

Or, we can provide the "TLAs" in three different ways: as string or code
values directly on the command line or as code values in a file:

Example with TLA as a string and as code:

$ jsonnet --tla-str name="My Name" --tla-code port=9999 a.jsonnet

{
   "name": "My Name",
   "port": 9999
}

Example with TLA as code on CLI and from yet another file that gives
multiple port numbers as a JSON array:

$ cat ports.json

[1234, 5678]

$ jsonnet --tla-code name='"My Name"' --tla-code-file port=ports.json a.jsonnet

{
   "name": "My Name",
   "port": [
      1234,
      5678
   ]
}