add infra_swarmbucket config for infra_paths module to override
git_cache. The value of path_config property value will be specified by
kitchen.

This will still delete cache every time. The goal of this change is to
fix build.git swarmbucket builds.

TBR=dnj@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2176913002