improvement: modifying the cli tool so that it can accept API_URL

[peter-mcconnell]

What

This solves the problem where you may be running airbyte on an endpoint
that isn't http://localhost:8001

Additionally it improves the error experience when this script can not
reach the API, and it only performs this curl download attempt if the config file isn't already present.

How

Allows the user to override API_URL if they wish
Explicitly handles the curl error in a user-friendly way

Recommended reading order

1. x.java
2. y.python

Pre-merge Checklist

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New Connector

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[CLAassistant]

All committers have signed the CLA.

[jrhizor]

Thanks for the contribution @peter-mcconnell !