swarming: add hostname to user agent
swarming: add hostname to user agent

This is to find which bot still sends request to isolate server.

Bug: 1253199
Change-Id: Ie569643508cbb250635aa157e01a88d0ab94f93a
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/3184981
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

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