Goal:
It's easy to forget old bots. We should reinstall them frequently. Reporting when the OS was installed helps with managing this.

AI:
- Add code to guess the installation time on each supported OS.
- Add it to the bot state
- Add it as a Web UI column

There are edge cases:
- netbooted machines: what to report, the time the image was created, or then the host booted?
- MP recycled GCE VMs: should we report the base image date (I think so)? It's probably "fine" in the case where the bot ID is generated.

But in general this should still be quite useful.