Change "Whitelabel" verbage to "Domain Authentication"

[crweiner]

Issue Summary

According to the Docs, SendGrid wishes to move away from the term "Whitelabel" and replace it with other terms, such as "domain authentication", "link branding" and "reverse DNS".

Details:

In order to keep this verbiage consistent, this repo needs to update its terms and replace "Whitelabel" with the more specific terms as described in the Doc.

Some locations for this change include:
https://github.com/sendgrid/sendgrid-python/blob/master/USAGE.md#whitelabel
https://github.com/sendgrid/sendgrid-python/blob/master/use_cases/domain_whitelabel.md
https://github.com/sendgrid/sendgrid-python/blob/b51522dc1bb2e3dadd1067c84adb6e628b20db1a/examples/whitelabel/whitelabel.py
https://github.com/sendgrid/sendgrid-python/blob/cd31fc8bc78b807b98e04120e42218e78ce7dbd4/use_cases/README.md

I'll be happy to try and tackle this set of changes. 👍