ch_integration_tests: Rely on Ubuntu Focal custom image

It's more reliable to use the custom image that we have on our Azure
storage account rather than pull everytime from the latest Focal release
from the Canonical repository.

Also, our custom image has been slightly cleaned up in order to prevent
services like snapd and pollinate to run and potentially cause some
troubles.

Signed-off-by: Sebastien Boeuf <[email protected]>

Author: Sebastien Boeuf

ch_integration_tests/run_tests.sh

@@ -13,8 +13,8 @@ if [ ! -f "$FW" ]; then
     popd
 fi
 
-FOCAL_OS_IMAGE_NAME="focal-server-cloudimg-amd64.img"
-FOCAL_OS_IMAGE_URL="https://cloud-images.ubuntu.com/focal/current/$FOCAL_OS_IMAGE_NAME"
+FOCAL_OS_IMAGE_NAME="focal-server-cloudimg-amd64-custom-20210407-0.qcow2"
+FOCAL_OS_IMAGE_URL="https://cloudhypervisorstorage.blob.core.windows.net/images/$FOCAL_OS_IMAGE_NAME"
 FOCAL_OS_IMAGE="$WORKLOADS_DIR/$FOCAL_OS_IMAGE_NAME"
 if [ ! -f "$FOCAL_OS_IMAGE" ]; then
     pushd $WORKLOADS_DIR