Update US GovCloud AD endpoint for AZURE_ARM provider

[CrCliff]

[AZURE_ARM] Update AD endpoint for Azure US GovCloud

Description

The AZURE_ARM provider was using the incorrect Active Directory endpoint for the Azure US Government environment.

I was seeing the following error when attempting to use the provider with the environment.

cls = get_driver(Provider.AZURE_ARM)
driver = cls(
    cloud_environment="AzureUSGovernment",
    tenant_id="...",
    subscription_id="....",
    key="...",
    secret="...",
)
# BaseHTTPError: Confidential Client is not supported in Cross Cloud request.

The az cloud list command provides the current AD endpoint.

$ az cloud list
  {
    "name": "AzureUSGovernment",
    "endpoints": {
      "activeDirectory": "https://login.microsoftonline.us",
     // ....
    },
    // ...
  },

After updating the endpoint the service principal authenticates as expected.

Status

• done, ready for review

Checklist

• Code linting (required, can be done after the PR checks)
• Documentation
• Tests
• ICLA (required for bigger changes)

[micafer]

LGTM

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.34%. Comparing base (124c47d) to head (5767351).
⚠️ Report is 10 commits behind head on trunk.

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk    #2062   +/-   ##
=======================================
  Coverage   83.34%   83.34%           
=======================================
  Files         353      353           
  Lines       81769    81769           
  Branches     8993     8993           
=======================================
  Hits        68148    68148           
  Misses      10796    10796           
  Partials     2825     2825           

Files with missing lines	Coverage Δ
libcloud/common/azure_arm.py	73.02% <ø> (ø)

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