Currently if path_config is not specified, infra_paths
defaults to buildbot paths. We want to make this behavior explicit,
so we don't have to introduce a new "luci" path configuration which
tells infra_paths module to do nothing.

Buildbot now specifies path_config=buildbot since
https://chromium-review.googlesource.com/c/412029/ ,
so now we can remove this default.

This also affects all expectation files because this CL switches 
the default path configuration from Buildbot to LUCI.
In particular, it changes default git cache dir from [GIT_CACHE]
to [CACHE]/git.
Also code paths that work only in Buildbot environment has to 
have a test with path_config=buildbot.

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

Change-Id: Ifeabe13e69810dd24a6deb4830699f8c9fafafae
Reviewed-on: https://chromium-review.googlesource.com/411989
Commit-Queue: Nodir Turakulov <nodir@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>