ipa_certupdate: avoid classmethod and staticmethod
    
    Because classmethod and staticmethod are just fancy ways of calling
    plain old functions, turn the classmethods and staticmethods of
    CertUpdate into plain old functions.
    
    This improves readability by making it clear that the behaviour of
    the routines cannot depend on instance or class variables.
    
    Part of: https://pagure.io/freeipa/issue/6577
    
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>