Add other supporting options for configuring credentials for AWS processors

I was looking at https://issues.apache.org/jira/browse/NIFI-770 and looked at how the AWS processors credentials are currently configured. As a NFM you have a few options with the properties right now:
1) set basic, static credentials
2) set a credentials properties filepath
3) set neither, use anonymous credentials

I think it would be better if each AWS could rely on a ControllerService that returns `AWSCredentialsProvider` (instead of `AWSCredentials`) that gives all of the possible implementations that could be used, rather than relying on a static credentials. *Provider implementations can be refreshed and can also other more complicated implementations, but already have built in support for the Static and Properties file that are provided by NiFi today.

My thinking is that the controller service would be something like

public interface AwsCredentialsProviderService extends ControllerService {
AWSCredentialsProvider getCredentialsProvider();
}

and you could have `StaticAwsCredentialsProviderService`, `PropertiesFileAwsCredentialsProviderService`, and `AnonymousAwsCredentialsProviderService` to provide the functionality that is supported right now. Additional credential providers could be added later, as there a bunch more AWS provided versions that I think could fit in well.