Look at the current number of idle VMs during resize logic
Look at the current number of idle VMs during resize logic

targetSize includes all VMs that are being created, deleted, etc. By looking only at idle VMs we actually wait for them to be created before flooding GCE with another increase request.

BUG=620534

Review-Url: https://codereview.chromium.org/2163413002

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