Use aws-sdk v2 in the trusted advisor check

[manelmontilla]

This PR introduces the following modifications to the vulcan-aws-trusted-advisor check:

• Migrate the code to use the go AWS sdk v2. The v2 comes out of the box with a retrier that should prevent throttling errors.
• Avoid an out of bound access to a slice in some situations.
• Fixed some minor styling issues.
• Allow the check to get the AWS credentials from the process env vars, so it's easier to run it locally.