JCLOUDS-457: Add list operation

Now the BlobStore abstraction supports the list Operation
User metadata is initialized by default already: https://github.com/jclouds/jclouds/blob/master/core/src/main/java/org/jclouds/domain/internal/MutableResourceMetadataImpl.java#L41

Do we therefore need this? Or is it important that we are setting an _immutable_ map here?