Fallback to default credentials when access key / secret are not provided

[bep]

• Use default credentials chain when access key / secret are not provided (fix(s3deploy): use default credentials chain when access key / secret are not provided #510)
• Adjust test assertions

[bep]

@HunterLarco sorry for the delay, turns out the fix was easier than I thought, it was just a different error message needing a different assertions.

[HunterLarco]

Thanks for coming back to it! I appreciate the fix :)