Add master_cleaner tool.

Add the master_cleaner tool, which aims to clean production master
directories by removing unused builder directories and old "twistd.log"
files.

BUG=chromium:586543
TEST=None

Review-Url: https://codereview.chromium.org/2059833002
Cr-Mirrored-From: https://chromium.googlesource.com/infra/infra
Cr-Mirrored-Commit: 793ded1c26d30e59c68c83bc6a9fd7bdb327a312
1 file changed