Configure chronyd for RHEL 7/CentOS 7/Fedora

The default NTP client for Fedora and RHEL 7 is chrony, instead of
ntpd. Configuring ntpd for systems where chrony is installed leads
to some issues, so the proposed patch will properly configure
chrony on RHEL 7, CentOS 7 and Fedora, while keeping ntpd for
earlier releases.

Change-Id: Ifd59f67909094327cbed280767292a45b542da2a
Merge "Configure chronyd for RHEL 7/CentOS 7/Fedora"