ConfDB: Add helper function to get "subsections"
    
    The secrets database will have "subsections", ie sections that are in the
    "secrets" namespace and look like this: [secrets/<path>]
    
    This function allows to source any section under secrets/ or under any
    arbitrary sub-path.
    
    Related:
    https://fedorahosted.org/sssd/ticket/2913
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>