feat(core): add environments block to datasource configuration

[puneetbehl]

This will add datasource config for each environments (development, test, and production).

Fixes #373

[CLAassistant]

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1 out of 2 committers have signed the CLA.

✅ puneetbehl
❌ Weiqi Gao

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[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ puneetbehl
❌ Weiqi Gao

---

Weiqi Gao seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[jamesfredley]

@weiqigao your two commits on this PR were pushed with an email address that github does not recognize. Could you sign the CLA for that email address or add that email to the Github account? See above.