Added Chrony NTP support

[mpassalacqua]

Starting with Ubuntu 18.04 (Bionic), chrony becomes the NTP default service. ntpd is not installed by default.
Also the latest Juju ntp charm (for Bionic) deploy chrony instead of ntpd (https://jujucharms.com/ntp/) so Contrail 5 deployment via Juju fails.

ntpd is progressively replaced by chrony also on the other linux distributions (like RHEL) because of the Autokey protocol that has serious security issues.

This is a generic check (not dependent by Linux dist) to support trigger the NTP check depending by the running NTP service.

[pcarver]

Contrail/Tungsten Fabric does not accept GitHub pull requests. All changes should be submitted via the Gerrit code review tool. If you're not familiar with Gerrit, take a look at section 4 of https://github.com/tungstenfabric/docs/blob/master/Contributor/GettingStarted/getting-started-with-opencontrail-development.md for more info.

The key points are that you need to login to https://review.opencontrail.org with an Ubuntu One account in order to establish your SSH public key. You need to clone the repos from Gerrit rather than (or in addition to) GitHub (or manually add an additional remote to an existing repo rather than cloning if you prefer). And you need to install and use the "git review" extension to Git in order to push changes to Gerrit.