Support default aws region

The aws s3 support (jackrabbit-aws-ext module) only supports providing the credentials through a file and through an OSGi configuration. However, in general all aws related services provide more configuration values, especially when running aws it makes sense to support the metadata service.
I suggest therefore to use com.amazonaws.auth.DefaultAWSCredentialsProviderChain.
Similar goes for the region, when running in aws,
Region region = Regions.getCurrentRegion();
provides automatically the correct region.

An OSGi configuration can be used for compatibility or testing to override things