compute: manage force_raw_images parameter

In Nova, force_raw_images allows to force backing images to raw
format.
Some use-cases require to disable this feature.

Let's use nova::compute class to manage this option, and let at True
by default as this the case in Nova (Juno release).

Change-Id: I4d700204bfd1d297667303b11e572f24dc9af2fc
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge "compute: manage force_raw_images parameter"