chore(omaha_request_params): remove crufty logic

These special channels mean nothing to me.
feat(omaha_request_params): read /usr/share/coreos/update.conf

Use /usr/share/coreos/update.conf as a second source of info for the
SERVER/GROUP information.
Merge pull request #20 from philips/use-usr-share-coreos-update.conf

feat(omaha_request_params): read /usr/share/coreos/update.conf