Sets up the credentials for the AWS API calls. Must be called before doing any remote calls to an AWS API. The client loader, aws.load() may transparently call it if you provide it the credentials.

client.setCredentials(accessKeyId, secretAccessKey)

• 'accessKeyId' - together with the secretAccessKey represents the credentials pair provided by AWS.
• 'secretAccessKey'

If this helper isn't called before making a remote request to the chosen AWS API, then an error is thrown.

This method is chainable (aka it returns the client object).