Generate random request ID base string to avoid request ID reuse
Generate random request ID base string to avoid request ID reuse

Currently, if a MachineType has its target_size reduced and MachineLease entities are deleted, if the MachineType has its target_size increased again and the MachineLease entities with the same index are recreated, they will use the same request ID base string as before, and start the count over at 0, which was already used to lease machines from the Machine Provider.

BUG=609308

Review-Url: https://codereview.chromium.org/2623273010

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