Strip cacert.pem to a minimum list of certificates.
Strip cacert.pem to a minimum list of certificates.

Use the list at https://pki.google.com/roots.pem. The previous version had CAs
that are not to be trusted anymore.

R=vadimsh@chromium.org
BUG=

Review URL: https://codereview.appspot.com/185580043

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