Allow Use of EC2 Instance Credentials

[gblanck]

The initialization of the awsCredentials relies on configured access_key and secret_key:

String accessKey = settings.get("metrics.cloudwatch.aws.access_key");
String secretKey = settings.get("metrics.cloudwatch.aws.secret_key");
awsCredentials = new BasicAWSCredentials(accessKey, secretKey);

The AmazonCloudWatchClient provides constructors that will leverage a credential provider chain that loads the IAM role of the host EC2 instance. Thus simplifying configuration and taking advantage of key rotation provided by AWS.

Perhaps a solution with backwards compatibility would be to create an extension of AWSCredentialsProviderChain with two chain members:

1. A credential provider implementation that mimics the current behavior of looking up of values in elasticsearch settings (throwing an exception if none exist causing 2 to execute)
2. An instance of DefaultAWSCredentialsProviderChain