Free up & return machines.
Free up & return machines.

BUG=chromium:288209
R=davidjames@chromium.org
CC=xusydoc@chromium.org
TEST=local, slaves_cfg, masters_cfg and masters _tests.py

Review URL: https://codereview.chromium.org/23621032

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@222161 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build