Add watcher for maintainence metadata changes

[deekue]

• instances with GPUs get a 1hr notification window instead of 60s.
• You only get the 1hr notification window if you're watching instance metadata, so we need to launch a watcher from gcloudrig-boot.ps1.
• there's an example Python script.
• If we implement this in Powershell in gcloudrig.psm1 we won't have to push another file.

Originally posted by @deekue in #12 (comment)

[deekue]

got a prototype working in Powershell: metadata_watcher.ps1