Disk configuration reworking: backend part. [PRD-1085]

This PR contains api changes.
* all size calculations in megabytes
* added converter to simple format, in which we communicates with UI
* reworking fixture format

Need to implement in next PRs
* validation
* use correct terminology everywhere. Rename disks to spaces spaces can contain volume groups or disks, each disk can contain volumes
* fix TODOs and clean up the code