Modularize AWS Credentials provisioning

[jrnt30]

Similar to #74 we use an alternate method for providing AWS credentials to our application. Per AWS best practices, we use IAM roles to provide automatic rotation of the credential keys autoatmaically (http://docs.aws.amazon.com/AWSSdkDocsJava/latest/DeveloperGuide/java-dg-roles.html).

We are currently using the AWSCredentialsProviderChain with values similar to https://github.com/aws/aws-sdk-java/blob/master/src/main/java/com/amazonaws/services/s3/AmazonS3Client.java#L294

By abstracting out the actual credentials creation into a factory, it would allow a lot of customization and custom credentials schemes.

[kgjohnso]

+1
Would be nice to be able to easily use credentials from the IMDS - Instance Metadata Service.
http://docs.aws.amazon.com/AWSSdkDocsJava/latest/DeveloperGuide/java-dg-roles.html

[qc00]

+1 I have an implementation for this that uses AWSCredentialsProvider to replace the user name and password pair. This is necessary because the instance profile credentials changes regularly. Currently my code did it via reflection, but native support will be helpful.

[sksastry]

+1 Would be a great step toward AWS best practices http://java.awsblog.com/post/Tx27W1S3VWL232L/Credentials-Best-Practices. Thank you!