Modified to allow use of EC2 instance profile

If credentials are not provided, the plugin will attempt to use an EC2
instance profile to authenticate to S3.

This required:

* Creating a new CredentialsProvider that will offer either the credentials provided in config or the credentials obtained via InstanceProfile.
* Upping aws-java-sdk version to latest
* Making access key and secret access key config parameters optional.

During the course of testing I also found that if I had not provided a value for health check period, the plugin would not successfully activate.  I changed this option to be required.