Keep credential provider chain

[MerlinDMC]

This PR will change the way the increased timeout for the AWS.EC2MetadataCredentials is set.

Currently this library is hard overriding anything that might have been set as the credentials source by the surrounding application and/or environment.
It makes it impossible to provide credentials via a configuration file in ~/.aws or environment variables not only for the kinesis stream writer but for the whole application using the KinesisStream class.

This behaviour was introduced by removing the configuration gate params.getCredentialsFromIAMRole here

A minimal demo for the problem using environment variables:

const assert = require('assert');

process.env.AWS_ACCESS_KEY_ID = 'dummy';
process.env.AWS_SECRET_ACCESS_KEY = 'invalid';

const AWS = require('aws-sdk');

assert(AWS.config.credentials instanceof AWS.EnvironmentCredentials,
  'Unexpected provider before KinesisWritable');

const KinesisWritable = require('aws-kinesis-writable');
new KinesisWritable({ streamName: 'invalid' });

assert(AWS.config.credentials instanceof AWS.EnvironmentCredentials,
  'Unexpected provider after KinesisWritable');