[cv] Misc fixes to luci-cv
[cv] Misc fixes to luci-cv

Minor simplifications and usage fixing.

R=yiwzhang

Change-Id: I3165316c5a47d5bd5e3b00b0a33feb7bccc44400
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3206730
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Commit-Queue: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>

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