Currently, "bot_update" relies on a BuildBot cleanup mechanism and, to a
lesser extent, the standard BuildBot directory layout. Both of these are
problematic when projecting it into other circumstances, notably
"remote_run" and LUCI.

Have "bot_update" handle its own cleanup. It will now choose a cleanup
directory within the hierarchy of its checkout, and explicitly purge it
prior to execution if it exists. This enforces its expected behavior in
all circumstances and removes its expectations of the greater checkout
layout.

Export "cleanup_dir" via "infra_paths" to point to "build.dead" when
running on BuildBot builds. Otherwise, it is a default directory which,
on Kitchen, is ephemeral by design.

BUG=chromium:725631
TEST=expectations

Change-Id: I664434c542a25aaa7ff3eac216208a2425730fde
Reviewed-on: https://chromium-review.googlesource.com/528057
Commit-Queue: Daniel Jacques <dnj@chromium.org>
Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>