Secrets: Add local container entries support
    
    The protocol requires a user to creat a container efore trying to store
    an entry in it. Do the same in the local provider so that no surprises
    arise when admins route request to a remote storage server.
    
    Signed-off-by: Simo Sorce <simo@redhat.com>
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>