Add support to provision api staging machines through prometheus

ping @pedro.

This adds 3 new commands to walle:

- walle staging me: Creates a new staging api for the user, if he doesn't have one
- walle what's on staging: Shows a list of all the staging environments
- walle destroy staging: Destroys the staging server for the user

I need to add PROMETHEUS_URL to the .env. I'll do once you approve it.