swarming: Remove sticky bits from caches when calling make_tree_deleteable().
swarming: Remove sticky bits from caches when calling make_tree_deleteable().

If a file/dir is owned by root and has the sticky bit set, only root can
remove it. If such a file is present in a bot's cache, the bot is unable
to delete that cache if the bot runs as non-root.

This change will remove all sticky bits from caches when the bot tries
deleting things, which should allow it to do so.

R=maruel

Bug: 919599
Change-Id: Iba741fc7a2935d092dbed7b1de6d8bb1b578a04b
Reviewed-on: https://chromium-review.googlesource.com/c/1399144
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py