Provide a method for retrieving on-disk metadata

We hide the internal dictionary for the metadata providing a method to
retrieve it to abstract away the implementation details of how
DiskFile object provides and maintains that metadata.

This is in anticipation of the DiskFile API refactoring.

Change-Id: I1c0dc01a4680bd435512405e2d31fba24421720a
Signed-off-by: Peter Portante <peter.portante@redhat.com>
Merge "Provide a method for retrieving on-disk metadata"