Adapt cert-find performance workaround for users
    
    ipa cert-find --users=NAME was slow on system with lots of certificates.
    User certificates have CN=$username, therefore the performance tweak
    from ticket 7835 also works for user certificates.
    
    Related: https://pagure.io/freeipa/issue/7835
    Fixes: https://pagure.io/freeipa/issue/7901
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>