Use the configured target size as the default, not the current target size
Use the configured target size as the default, not the current target size

machine_type.target_size gets changed by the schedule. When defaulting we need to use the original number from config.target_size.

BUG=694876

Review-Url: https://codereview.chromium.org/2777473002

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