AWS authentication through the regular aws cli authentication

[rafaelromcar-parabol]

Issue - Enhancement

https://docs.aws.amazon.com/sdk-for-javascript/v3/developer-guide/getting-your-credentials.html

Today we require to pass explicit variables for AWS authentication (only used when filestore provider is set to s3) but that's not a best practice as it does not allow setting credentials through service accounts in Kubernetes or other industry standard methods.

Parabol should allow AWS authentication the same way the AWS NodeJS SDK allows it. In short: allowing to use local configuration and service accounts and discourage explicit credentials from AWS keys.

Acceptance Criteria (optional)

Users can:

• Do this
• Do that
• Cannot do that

Estimated effort: X points (see CONTRIBUTING.md)