systemd restart loop delays shutdown

**Image I'm using:**
`v0.11.7`

**Issue or Feature Request:**
When testing locally in `aws-dev`, I issued an `apiclient reboot` command and hit an error where this pair of messages spammed the console during system shutdown:
```
[ 135.289329] host-containers@admin[1168]: serial-getty@ttyS0.service has no holdoff time, scheduling restart.
[ 135.290142] host-containers@admin[1168]: Stop job pending for unit, delaying automatic restart.
```

I've also heard scattered reports that enabling the admin container can cause delays when scaling in a cluster, and I suspect this might be the culprit.

systemd fixed this upstream in https://github.com/systemd/systemd/commit/c972880640ee19e89ce9265d8eae1b3aae190332 which we should add to the systemd patches here.