Add package and instance id to CIPD user agent.
Add package and instance id to CIPD user agent.

R=dnj@chromium.org, vadimsh@chromium.org
BUG=663843

Review-Url: https://codereview.chromium.org/2542513002

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go