tryserver module's purpose is to provide high level API for patches.
Update it to use buildbucket.build.input.gerrit_changes[0].
Add helper properties for gerrit repo URL and gerrit ref.

This will be used by both bot_update and gclient.

R=hinoka@chromium.org, tandrii@chromium.org

Bug: 694348
Change-Id: I7eb416e75737310404c4354fa680fd3499eddaa7
Reviewed-on: https://chromium-review.googlesource.com/1211653
Commit-Queue: Nodir Turakulov <nodir@chromium.org>
Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>