[WIP] Optionally allow users to set NTP servers for the cluster

[lblackstone]

NTP servers on Container Linux default to CoreOS time servers,
which may not be accessible on private cluster deployments.
Allow users to set a list of primary NTP servers with the CoreOS
servers as the default fallback.

Fixes #1866

[coreosbot]

Can one of the admins verify this patch?

[coreosbot]

Can one of the admins verify this patch?

[lblackstone]

@s-urbaniak @alexsomesan I've implemented this for OpenStack so far. If this approach looks good, I can add the other platforms as well.

[mxinden]

We did some changes (#2082) to the testing process. Please rebase on to current master, so that the basic-tests PR status is reported correctly.

[lblackstone]

@coreos/team-installer Could somebody take a look at this WIP and let me know if this approach is ok?

[s-urbaniak]

@lblackstone thanks a lot for the contribution!

/cc @robszumski for the product side of things
/cc @crawford @lucab if this is the right approach from the perspective of CL

[lucab]

I'd suggest to avoid installing the config file if the list is empty, as this just hardcodes the same default values but in a way that can't be changed in the future.

[lucab]

This should probably go in its own dropin (i.e. /etc/systemd/timesyncd.conf.d/10-tectonic-installer.conf) and be skipped if the terraform configuration value is not set by the user (to avoid bloating userdata and hardcoding defaults in config).

[lblackstone]

Addressed @lucab comments. Drop-in file is now only installed if the user specifies NTP servers.

[crawford]

I thought the installer was moving away from including this sort of configuration in favor of it moving into a cluster config that gets applied at runtime instead.

[lblackstone]

@crawford Do you have any further information on the direction you mentioned? Happy to revise as needed, but I hadn't seen any public-facing docs to that effect.

/cc @s-urbaniak

[crawford]

@lblackstone Unfortunately, I don't have any more info at the moment. This repo will be undergoing a pretty massive refactor over the next couple months (all driven by CoreOS-internal needs). Hopefully by the end of this work, this repo will feel a bit more like an open source project.

[lblackstone]

#2747 finished out this work for the track-1 branch. Closing this PR, as that feature can be cherry-picked later if desired.