Switch vexxhost to RAW image format

The VEXXHOST public cloud uses Ceph-backed instances
for it's infrastructure.  When using QCOW2 images,
the content of the image is imported on every boot
of the instance, in some cases almost ~15GB write.

When a RAW image is uploaded, OpenStack Nova will
use copy on write to create a new copy of the image
very quickly and efficently, speeding up boot times
and reducing load.

Change-Id: I0210b70b6812588f3e5dcad10c08d4ee8be29be3

nodepool/nodepool.yaml

@@ -1227,6 +1227,7 @@ providers:
     boot-timeout: 120
     max-servers: 10
     rate: 0.001
+    image-type: raw
     images:
       - name: ubuntu-trusty
         min-ram: 8000