EC2 Inventory Boto Profile Support

This PR effectively combines #5987 and #8582. It also fixes the merge conflicts in the two PRs and standardizes the option names.

I also removed the environment variable support as boto has since added AWS_PROFILE (starting with version 2.29) which accomplishes the same thing and is consistent with AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY.

There is a minor inconsistency right now with the cache directory names. If you use the AWS_PROFILE environment variable, your cache directory name will not change. However, if you use the CLI option or the config option, the cache directory will be named after the profile. We could check the AWS_PROFILE for naming, but the behavior is debatable and I've started a discussion on the ansible-devel mailing list.
Nice catch. I'll fix this and update the PR.