Provide check for clock sync on gitaly nodes

It is required that Gitaly servers and clients utilize NTP to keep their clocks in sync.

We provide a check for clock sync between Geo nodes as part of gitlab-rake gitlab:geo:check.

It would be helpful if the Gitaly health check task that is run as part of gitlab-rake gitlab:check task run on the app server alerted when there is clock skew on a Gitaly node.

This would potentially speed up diagnosis of the issues that arise if Gitaly node clocks drift apart.