cache: support MinFreeSpace
cache: support MinFreeSpace

Bug: 932396
Change-Id: Ic103bf894419104acbb7ddb7b1c3a1fdf0ba4720
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1871531
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <5d95c5c75e3d691d9c77888fff4498e04f8b7157@google.com>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go