Add XCode searching code to platforms.osx.
Add XCode searching code to platforms.osx.

Migrate code that was put into a bot_config.py into a more general place.

R=vadimsh@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1309033002

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py