fix: multi model integration test to create ECR repo with unique names to allow independent parallel executions

[dlragha]

fix: multi model integration test to create ECR repo with unique names to allow independent parallel executions

Issue #, if available:

Description of changes:
The multi model tests create an ECR repo with a multi-model image as part of integration test setup and deletes it during cleanup. Running multiple multi model tests in parallel was causing the setup and cleanup of these ECR repos to conflict with each other resulting in test failures. This PR has the following changes:

1. Modified the logic to create ECR image with "unique" name by including a timestamp in it
2. Added retries to docker image push code so that we do don't give up on intermittent connection errors due to running multiple docker push commands at the same time.
3. Improvements to ECR image creation/deletion exception handling

Testing done:

1. Ran multi model integration tests on personal AWS account successfully:
   $ tox -e py36 -- tests/integ/test_multidatamodel.py -m "not local_mode" -n 48 --reruns 0 --reruns-delay 5 --durations 50 --boto-config '{"region_name": "us-east-2"}'
2. Style checking, etc:
   $ tox -e flake8,pylint,twine,black-check

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• I have added tests that prove my fix is effective or that my feature works (if appropriate)
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• I have updated any necessary documentation, including READMEs and API docs (if appropriate)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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