[milo] Set min active instances
[milo] Set min active instances

Add in automatic scaling factors for Milo.

Milo's default service generally has about 9-12 instances active, so this just
codifies the minimum, and there shouldn't be a difference.

Bug:806700
Change-Id: Ifbd21dc2697b12f9784b0a7bc90ada851b20d777
Reviewed-on: https://chromium-review.googlesource.com/892019
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Ryan Tseng <2190fe9a39b4920ec7bd59c848ec5cccd163056f@chromium.org>

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