Allow setting raid level/config per partition

[SharkWipf]

Currently, you can only select one raid level for all following partitions, despite each partition being created as a separate raid device.
It would be great to have the option to have different raid levels per partition, i.e.:

• Raid 1 for the server;
• Raid 1 for a guest images partition/volume;
• No raid for (duplicated) swap partitions, since these stripe by default;
• Raid 0 for a volume dedicated to swap partitions for VMs, since those don't really benefit from raid 1 anyway.

Currently, to achieve this, we have to manually adjust the partitions after the installation is done.

[cyberplant]

Please check my PR #85