Disks refactoring.

unit-tests && by-hand

Contains
[Multiboot](https://github.com/Mirantis/fuelweb/pull/1258)
[Disk configuration reworking: JS part](https://github.com/Mirantis/fuelweb/pull/1263)
[Disk configuration reworking: backend part. [PRD-1085]](https://github.com/Mirantis/fuelweb/pull/1262)