We cannot easily move git cache location on all buildbot bots, but we
need to define a new value for LUCI.
Introduce infra_paths.default_git_cache_dir which must be used instead
of using path['git_cache'] directly.

R=iannucci@chromium.org, martiniss@chromium.org
BUG=662586

Change-Id: I5244d7a761da94858df798abbd423c85f55936af
Reviewed-on: https://chromium-review.googlesource.com/411993
Commit-Queue: Nodir Turakulov <nodir@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>