Detecting Regional Endpoint from env var

[rayluo]

• This PR adds back the behavior of detecting region based on env var REGION_NAME. Both short name (as eastus) or long name style (as West Us 2) are supported.

• This time, we also make sure the entire regional endpoint behavior requires opt-in (as reiterated by this internal design), by either

  ConfidentialClientApplication(..., azure_region='westus', ...)

  or

  ConfidentialClientApplication(..., azure_region=ConfidentialClientApplication.ATTEMPT_REGION_DISCOVERY, ...)

  The default behavior does not utilize regional endpoint.

  ConfidentialClientApplication(..., azure_region=None, ...)

  In other words, the re-introduction of reading REGION_NAME env var would not regress The REGION_NAME env var should be prefixed with MSAL_ #382.