Update debootstrap during the Kokoro build

- Debootstrap on Kokoro looks to have a bug that prevents our build from
suceeding, with errors about systemd pre-dependencies. Looks like this
was an actual bug in debootstrap: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=857338
- Update to the debootstrap from Ubuntu Xenial during our build, as the
bug should be fixed in that version.

Change-Id: I3c1316a98ddc654a0bf12aa755cbe4a7d11fa24b
1 file changed