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
Committed: https://github.com/luci/luci-go/commit/766ec084433f12dfd18a392c29789e412d59c7da

[iannucci, 2016-11-30 01:43:05]

This ends up sending a UserAgent like `cipd 1.3
(infra/tools/cipd/mac-amd64@dd7ffbc69f5a716c78e0e2fad6a8f64e52bad7b1)`

[Vadim Sh., 2016-11-30 01:57:27]

lgtm

[iannucci, 2016-12-02 00:11:19]

The CQ bit was checked by iannucci@chromium.org

[commit-bot: I haz the power, 2016-12-02 00:11:30]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-12-02 00:18:24]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1480637479214380, "parent_rev":
"2b85e2e02789e4f20bfd0200569382e0faeb872c", "commit_rev":
"766ec084433f12dfd18a392c29789e412d59c7da"}

[commit-bot: I haz the power, 2016-12-02 00:18:28]

Description was changed from

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

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

to

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

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

Committed:
https://github.com/luci/luci-go/commit/766ec084433f12dfd18a392c29789e412d59c7da
==========

[commit-bot: I haz the power, 2016-12-02 00:18:30]

Committed patchset #1 (id:1) as
https://github.com/luci/luci-go/commit/766ec084433f12dfd18a392c29789e412d59c7da