Allow refclock configuration parameter

[islepnev]

Since Linux-4.11 ptp_kvm kernel module is available allowing use of kvm infrastructure as a PTP clock.
The following line is required in chrony.conf:
refclock PHC /dev/ptp0 poll 3 dpoll -2 offset 0
It could be used in more generic way to add GPS and other hardware reference clocks too.

Since there's no generic option to add arbitrary data to chrony.conf, patching puppet-chrony code is required.

See also article: https://opensource.com/article/17/6/timekeeping-linux-vms