Change AWS default credential option to access keys

[kaiyan-sheng]

What does this PR do?

This PR changes AWS default credential option to access keys. When running on Docker, AWS credential file needs to be provided via volume mount and this is not shipped with elastic agent by default. Therefore, we should recommend users to use access key, secret access key and/or session token as default credential option.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• If I'm introducing a new feature, I have modified the Kibana version constraint in my package's manifest.yml file to point to the latest Elastic stack release (e.g. ^7.13.0).

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• Start Time: 2021-07-13T14:24:46.584+0000

• Duration: 15 min 30 sec

• Commit: 9e617d6

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Test	Results
Failed	0
Passed	247
Skipped	0
Total	247

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