Use application credentials

[tomasgonzalo]

Made a few changes:

• Now authentication is enabled by using application credentials from either an OpenRC file or a clouds.yaml file
• Backup and restoration of volume-backed instances is enabled
• Backups of instances is enabled (in addition to snapshots)

This has been tested in the old cloud and the new cloud under EUMETSAT.

Pending:

• Tests on the ECMWF cloud, doing those right now
• UX improvement changes suggested by @slothkong, will do this asap

Not sure who will review this @pacospace @slothkong ?

[slothkong]

Hi @tomasgonzalo,
I can provide feedback on short notice. Just assign me when you consider the development done on your end :)