Check for disabled masters or builders.
Check for disabled masters or builders.

Not all masters work with bot_update yet, so now just make enabled
the default but allow to specify a blacklist.

BUG=504679
TBR=phajdan.jr@chromium.org

Review URL: https://codereview.chromium.org/1212303002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@295851 0039d316-1c4b-4281-b951-d872f2087c98

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