Correct the lookup of the health endpoint to avoid caching when the lambda is warm.

[grahamhar]

This is an attempt to fix the issue reported in issue: https://github.com/aws-samples/aws-health-aware/issues/16

Description of changes:

Setting the boto config via a lookup done at the global level of file means that a change in the active region would not be picked up if the lambda was warm. I've added test cases as I prefer to work on code with tests, feel free to remove them if prefer.

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