compute: fix unknown flavor handling

In real life, it happens that old VM run with flavors that have been
deleted. This wasn't handled properly.

This fixes bug #1131751

Change-Id: I11f32946394f0245f1696f29ff625b53a1a1cdb8
Signed-off-by: Julien Danjou <julien@danjou.info>
Merge "compute: fix unknown flavor handling"