Set root mount options on debian buster image

This is an attempt to get better fs IO performance out of our images.
Start with this on the buster image as this gives us a low impact test
if things go wrong, but also provides a system similar ish to ubuntu.

Specifically we disable barriers and set noatime as we don't care about
data reliability after crashes on our test nodes (they are single use)
and atime shouldn't matter for our CI system.

Change-Id: Ied44590c6260691a738ed105f8da0f4765b6c162

nodepool/nodepool.yaml

@@ -214,6 +214,25 @@ diskimages:
       GIT_HTTP_LOW_SPEED_TIME: '300'
       GIT_HTTP_LOW_SPEED_LIMIT: '1000'
       DIB_SHOW_IMAGE_USAGE: '1'
+      DIB_BLOCK_DEVICE_CONFIG: |
+        # Default single partition loopback
+
+        - local_loop:
+            name: image0
+
+        - partitioning:
+          base: image0
+          label: mbr
+          partitions:
+            - name: root
+              flags: [ boot, primary ]
+              size: 100%
+              mkfs:
+                mount:
+                  mount_point: /
+                  fstab:
+                    options: "defaults,nobarrier,noatime"
+                    fsck-passno: 1
   - name: fedora-29
     pause: false
     elements: