[milo] validate service (global) config file settings.cfg.
[milo] validate service (global) config file settings.cfg.

R=hinoka

Bug: 796317
Change-Id: Ic72774f69c6d817ffb9ab60b4154a78ea617751f
Reviewed-on: https://chromium-review.googlesource.com/1060248
Reviewed-by: Ryan Tseng <2190fe9a39b4920ec7bd59c848ec5cccd163056f@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>

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