Use get_gerrit_patch_root in bot_update.ensure_checkout.

This fixes some tests expectations that expected CI builds to pass --patch_root
flag.

Also unify "root" and "patch_root" variables in ensure_checkout. Simpler.

This CL requires nontrivial rolls in all downstream packages because all
packages have incorrect expectations and/or tests.
In particular, this removes --patch_root flag from non-tryjob test
cases. Some tests are supposed to test tryjob scenario, but they use
properties.generic instead of properties.tryserver (or buildbucket.try_build)

Recipe-Nontrivial-Roll: build
Recipe-Nontrivial-Roll: build_limited_scripts_slave
Recipe-Nontrivial-Roll: infra
Recipe-Nontrivial-Roll: release_scripts
Recipe-Nontrivial-Roll: skiabuildbot
Bug: 694348
Change-Id: Ibc594fa42e3b4d2823672b41dda2d97154b10d38
Reviewed-on: https://chromium-review.googlesource.com/1231280
Commit-Queue: Nodir Turakulov <nodir@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>