coreos-base/oem-gce: keep unit after its run

Otherwise it gets restarted a few times, which displays this line in the
logs:
```
Nov 30 13:28:41.819250 enable-oslogin[1232]: /etc/pam.d/sshd already exists. Not enabling OS Login
```

Signed-off-by: Mathieu Tortuyaux <mtortuyaux@microsoft.com>

Author: tormath1

sdk_container/src/third_party/coreos-overlay/coreos-base/oem-gce/files/units/oem-gce-enable-oslogin.service

@@ -8,6 +8,7 @@ Before=sshd.socket
 
 [Service]
 Type=oneshot
+RemainAfterExit=true
 
 ExecStart=/oem/bin/enable-oslogin