Make addClientId and addEnvironment to be configurable for study telemetry

[motin]

Rationale: some studies benefit of even more privacy and leaner ping envelopes.

TODO:

• Clone https://github.com/gregglind/shield-study-schemas to the mozilla org
• Make clientId a non-required attribute in the shield-study schemas
• Send a PR to update https://github.com/mozilla-services/mozilla-pipeline-schemas/blob/master/README.shield.md with up to date information
• Send a PR against https://github.com/mozilla-services/mozilla-pipeline-schemas with the updated shield-study schemas