Previously, the service file's name is 'k8s_${POD_UID}.service',
which means we need to `systemctl daemon-reload` if the we replace
the content of the service file (e.g. pod is restarted).
However this makes the journal in the previous pod get disconnected.
This PR solves the issue by using the unique rkt uuid as the service
file's name. After the change, the service file's name will be:
'k8s_${rkt_uuid}.service'.
f64c47ef80