ipaserver/install/cainstance.py: unlink before creating new file in /tmp
    
    When fs.protected_regular=1 root cannot open temp files that
    are owned by other users read-write.
    So unlink temporary file before shutil.copy to it.
    
    Fixes: https://pagure.io/freeipa/issue/7907
    Signed-off-by: François Cami fcami@redhat.com
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>