[UX] CLI for creating datasources.json?

[dhmlau]

Following the example-getting-started to create my own app, I'm trying to create a datasource.json that's connecting to a Cloudant database (it could be other kind of db).

Currently what I've done is to go back to LB3, run the command lb datasource to generate the datasources.json and copy this file back to my LB4, so that I can have a similar file like https://github.com/strongloop/loopback-next/blob/master/packages/example-getting-started/config/datasources.json.

Since we'll be using the legacy juggler as a near-term solution, can we bring this datasources.json generation experience to LB4?

cc @kjdelisle @raymondfeng

[dhmlau]

or using the DataSource class is the recommended route?

[dhmlau]

duplicate of #1225