function finish(err) {
	      callback(err, err ? null : self.credentials);
	    }
Loads credentials from the configuration object. This is used internally
by the SDK to ensure that refreshable {Credentials} objects are properly
refreshed and loaded when sending a request. If you want to ensure that
your credentials are loaded prior to a request, you can use this method
directly to provide accurate credential data stored in the object.

@note If you configure the SDK with static or environment credentials,
  the credential data should already be present in {credentials} attribute.
  This method is primarily necessary to load credentials from asynchronous
  sources, or sources that can refresh credentials periodically.
@example Getting your access key
  AWS.config.getCredentials(function(err) {
    if (err) console.log(err.stack); // credentials not loaded
    else console.log("Access Key:", AWS.config.credentials.accessKeyId);
  })
@callback callback function(err)
  Called when the {credentials} have been properly set on the configuration
  object.

  @param err [Error] if this is set, credentials were not successfully
    loaded and this error provides information why.
@see credentials
@see Credentials