Automatic merge from submit-queue

rkt: Get logs via syslog identifier

This change works around https://github.com/coreos/rkt/issues/2630
Without this change, logs cannot reliably be collected for containers
with short lifetimes.

With this change, logs cannot be collected on rkt versions v1.6.0 and
before.

I'd like to also bump the required rkt version, but I don't want to do that until there's a released version that can be pointed to (so the next rkt release).

I haven't added tests (which were missing) because this code will be removed if/when logs are retrieved via the API. I have run E2E tests with this merged in and verified the tests which previously failed no longer fail.

cc @yifan-gu