Add ephemeral flag to Collector enrollment tokens

The idea is to have an "ephemeral" flag on enrollment tokens. Instances enrolled with this token are marked as "ephemeral" and will be cleaned up earlier than non-ephemeral instances. This can be useful in environments with short-lived instances like container.

Should we implement alerts for disappearing nodes, ephemeral nodes would not be included in those alerts.

Alternative: Make cleanup intervals configurable per fleet. (same with alerting)