timesync: Make delaying attempts to contact servers configurable

```
❯ ssh sus@xx.xx.xx.xx
Last login: Sat Nov 14 17:32:08 2020 from 10.104.45.138
 17:36:19 up 0 min,  0 users,  load average: 0.00, 0.00, 0.00
> systemd-analyze blame
Bootup is not yet finished (org.freedesktop.systemd1.Manager.FinishTimestampMonotonic=0).
Please try again later.
Hint: Use 'systemctl list-jobs' to see active jobs
> systemd-analyze blame
43.954s systemd-time-wait-sync.service
 1.969s systemd-networkd-wait-online.service
 1.559s cloud-init-local.service
 1.039s cloud-init.service
  414ms cloud-final.service
  387ms dracut-initqueue.service
  382ms initrd-switch-root.service
  380ms cloud-config.service
  198ms systemd-journal-flush.service
  136ms systemd-udev-trigger.service
  115ms initrd-parse-etc.service
   97ms systemd-timesyncd.service
   84ms systemd-journald.service

```

After made it configurable and set to 5s

```
❯ ssh sus@xx.xx.xx.xx
Last login: Sat Nov 14 18:41:42 2020 from 10.104.45.138
 18:42:36 up 0 min,  0 users,  load average: 0.16, 0.03, 0.01
> systemd-analyze blame
10.450s systemd-time-wait-sync.service
 8.303s systemd-networkd-wait-online.service
 1.621s cloud-init-local.service
 1.068s cloud-init.service
```