Simplify handling of our custom certificates
    
    The property loader is an overly-complex solution to cache the results
    of getting a property.
    
    In the case of our certificates, the call is not very expensive so we
    don't even need to cache it at all.
    
    However, given that pyrpkg now has a cached_property decorator which
    does just that, we might as well use it.
    
    Signed-off-by: Mathieu Bridon <bochecha@daitauha.fr>