provider/aws: Fix issue updating Elastic Beanstalk Environment variables

[catsby]

There is a bug in how aws_beanstalk_environment handles updating environment variables, specifically when modifying more than a single one. When more than one is modified, a replace operation should occur. Because of how Beanstalk handles things, we don't do our normal remove all these then add all these, as we normally do.

There is logic in place to handle this, but the test case was too simple and did not test modifying a variable, only adding or deleting. When modifying, we need to essentially do a PUT. Due to an error in the logic, modifying a variable resulted sending the new value while also telling the Service to remove it completely.

This PR changes the logic to remove items that are both being added and deleted from the slice of items to remove, instead of trying to build up a new set of removals. I've also modified an existing test to verify the behavior.

Fixes #8742

[jbardin]

👍

[ghost]

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