[luci_notify] Refactor Builder update to be less fragile
[luci_notify] Refactor Builder update to be less fragile

This change refactors the way Builders are updated so that only the
fields that actually need to get updated from the new configuration are
updated, and not the current dance we're doing where we end up trying to
explicitly preserve existing fields.

Change-Id: I1418a5534c4773cd619c4691e35c2a4287a51a5a
Reviewed-on: https://chromium-review.googlesource.com/1103232
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>

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