connector:* normalize connector config and show warning messages if users use old config specification

[muga]

Data connector can acquire 3 versions of config specifications from td CLI because td API server normalizes all of them. But we should move server-side normalization code to td CLI side. td CLI should send latest (3rd) config only to td API server. And td CLI can show warning messages if users still use old versioned config spec.

The following is 3 types config specification:

• 1st version of config specification: doesn't include in key like:

type: s3
bucket: my_bucket
path_prefix: path/to/prefix
...

• 2nd version: includes config key like:

config:
  in:
    type: s3
    bucket: my_bucket
    path_prefix: path/to/prefix
...

• connector: remove unnecessary config key #79 3rd version: Embulk config specification like:

in:
  type: s3
  bucket: my_bucket
  path_prefix: path/to/prefix
...