sanitise depcfg in exported file

As of now, depcfg looks like the following:

 

"depcfg": {

    "buckets": [

      {

        "alias": "tgt",

        "bucket_name": "target"

      },

      {

        "alias": "src_logs",

        "bucket_name": "logs"

      }

    ],

    "metadata_bucket": "metadata",

    "source_bucket": "logs"

  },

The format above has the following issues:

1. Not readable
2. Not future-proof

Recommendation

1. Move metadata_bucket and source_bucket to TLDs
2. Call the structure as "bindings" and use the following format:

"bindings" : {

[

  {"type":"alias", "bucket_name":"<src_bucket>", "value":"src_bucket"},

  {"type":"alias", "bucket_name":"<tgt_bucket>", "value":"tgt_bucket"},

  {"type":"key", "key_name":"<key_name>", "value":"key_value"},

]

}

It is important that this be done for beta, as otherwise the current exported structure will have to be supported for future, making it cumbersome during upgrades.