update_engine: Log the rootfs and stateful partition's lsb-release

postinst cannot not do this anymore, so we need to log it in the UE itself.

BUG=chromium:786225
TEST=cros_flash two times and made sure they are logged.

Change-Id: I2fbc593f55e6425127de283c78e4170460bac0d9
Reviewed-on: https://chromium-review.googlesource.com/791515


Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Alex Deymo <deymo@google.com>