botmap fix for builders.pyl masters.
botmap fix for builders.pyl masters.

Because new builders.pyl masters generate slave config with 'builder_name'
instead of 'builder', botmap didn't emit builders, preventing auto-config by
puppet.

R=vadimsh@chromium.org,dpranke@chromium.org,machenbach@chromium.org
BUG=553433

Review URL: https://codereview.chromium.org/1432953003

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@297504 0039d316-1c4b-4281-b951-d872f2087c98

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