Added floating builder utility module
Added floating builder utility module

Added a 'floating builder' utility module, which enables prioritized selection
of tagged slaves for builds.

BUG=386631
TEST=localtest

Review URL: https://codereview.chromium.org/343923003

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@280544 0039d316-1c4b-4281-b951-d872f2087c98

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