node.xml: improve ntpd security

the current configuration of ntpd on rocks is vulnerable to
NTP reflection attach:
http://www.symantec.com/connect/blogs/hackers-spend-christmas-break-launching-large-scale-ntp-reflection-attacks

We have a firewall in front of the NTPD on the frontend nevertheless
I see no reason why we shouldn't be using the default centos configuration
file which disable this vulnerability and it also disable many other
unecessary functions which can expose sensitive information

This patch tries to edit the configuration files which are shipped
with centos and not to overwrite them