[server] Very minor refactoring of server initialization.
[server] Very minor refactoring of server initialization.

There'll be more changes in initAdminPort later.

R=tandrii@chromium.org

Change-Id: Id20f6c982dc39ab0b333bb15d6e2f4cdbc00d19e
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1952152
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

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