storage: Add support for access to files using provided uid/gid
    
    To allow using the storage driver APIs to access files on various
    storage sources in a universal fashion possibly on storage such as nfs
    with root squash we'll need to store the desired uid/gid in the
    metadata.
    
    Add new initialisation API that will store the desired uid/gid and a
    wrapper for the current use. Additionally add docs for the two APIs.