Support the certmonger nss-user option
    
    Some certificate operations need to be executed as a specific
    user so that underlying files will have the correct ownership.
    certmogner normally runs as root. The nss-user option defines
    a user to switch to when saving NSS certifciates so if a
    software token (e.g. SoftHSM) then the files created will be
    owned by the token owner.
    
    Fixes: https://pagure.io/freeipa/issue/9273
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>