change: add MXNet configuration to image_uris.retrieve()

[laurenyu]

Issue #, if available:
#1464

Description of changes:
Because I got a little carried away with refactoring the tests, I'll update the MXNet test fixture usage in a separate PR

Testing done:
unit tests, linters

Merge Checklist

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General

• I have read the CONTRIBUTING doc
• I used the commit message format described in CONTRIBUTING
• I have passed the region in to all S3 and STS clients that I've initialized as part of this change.
• I have updated any necessary documentation, including READMEs and API docs (if appropriate)

Tests

• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have checked that my tests are not configured for a specific region or account (if appropriate)
• I have used unique_name_from_base to create resource names in integ tests (if appropriate)

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[sagemaker-bot]

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• CodeBuild project: sagemaker-python-sdk-notebook-tests
• Commit ID: f33b89c
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AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-zwei-pr
• Commit ID: f33b89c
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AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-local-mode-tests
• Commit ID: f33b89c
• Result: SUCCEEDED
• Build Logs (available for 30 days)

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