Add iburst parameter to ntp class

I used your puppet-ntp module to manage the ntp configuration on a server that was using the iburst mode in ntp.conf.  I added a parameter to control the addition of the iburst mode flag so that clients could choose to include it or not.

From man page:

"Most applications will probably want to specify the iburst option with the server command. With this option a volley of messages is exchanged to groom the data and set the clock in about ten seconds."

c.f. http://www.eecis.udel.edu/~mills/ntp/html/ntpd.html