Don't err when there are no declared instance group managers for a given template
Don't err when there are no declared instance group managers for a given template

BUG=609308
TBR=vadimsh

Review-Url: https://codereview.chromium.org/1955073003

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py