Don't always transact when attempting to create/update MachineLeases
Don't always transact when attempting to create/update MachineLeases

Check outside a transaction first.

BUG=609308

Review-Url: https://codereview.chromium.org/2624353002

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