Make user agent set via envvar, not CLI option

Make user agent set via envvar, not CLI option

R=dnj@chromium.org, vadimsh@chromium.org
BUG=663843
Committed: https://github.com/luci/luci-go/commit/3d891f6c0233be285a79cc123c98440df5e234ec

[Vadim Sh., 2016-11-30 00:25:22]

lgtm

[iannucci, 2016-11-30 00:27:34]

The CQ bit was checked by iannucci@chromium.org

[commit-bot: I haz the power, 2016-11-30 00:27:41]

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, 2016-11-30 00:34:31]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1480465654990700, "parent_rev":
"06da8fd9f80dffc72b5fa66c28226d237c285044", "commit_rev":
"3d891f6c0233be285a79cc123c98440df5e234ec"}

[commit-bot: I haz the power, 2016-11-30 00:34:33]

Description was changed from

==========
Make user agent set via envvar, not CLI option

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

to

==========
Make user agent set via envvar, not CLI option

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

Committed:
https://github.com/luci/luci-go/commit/3d891f6c0233be285a79cc123c98440df5e234ec
==========

[commit-bot: I haz the power, 2016-11-30 00:34:34]

Committed patchset #1 (id:1) as
https://github.com/luci/luci-go/commit/3d891f6c0233be285a79cc123c98440df5e234ec