use chrony (not ntpd) on RHEL- and SUSE-based systems
chrony has been in Ubuntu Universe since Trusty, and it may become the default in Ubuntu as soon as 18.04: https://bugs.launchpad.net/bugs/1744072
openSUSE and SLES are moving to chrony as well.
Chrony upstream has active development upstream and it would be great to save engineering, quality assurance and support resources by supporting the newer NTP client / server implementation. ntpd has security issues, described at https://chrony.tuxfamily.org/comparison.html, https://access.redhat.com/solutions/2070363, and https://lwn.net/Articles/735211/
We should switch our Ceph docs, Teuthology tests, and ceph-ansible to using chrony on RHEL-based OSes (maybe all OSes) instead of ntpd.