cv: accept clientID primarily for rpcexplorer/.
cv: accept clientID primarily for rpcexplorer/.

Also, hardcode the value of it for -dev for convenience.
It's also the one that will be used in prod, for the time being.

R=vadimsh, yiwzhang

Bug: 1139324
Change-Id: I828e5fd4d30bd656177e3f6942f93c6ff7c4cd13
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2493840
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Auto-Submit: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

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