chore: Update the BTRFS disk check for AIOs

The disk check for AIO deployments using BTRFS was breaking due to
a change in the command line utility output. This change updates
the commands we use to ensure that the output is usable.

Change-Id: I548ad1a4a0ee7527f7b5fcd871ed55c9bc5ce6e7
Signed-off-by: Kevin Carter <kevin.carter@rackspace.com>

tests/roles/bootstrap-host/tasks/check-requirements.yml

@@ -47,7 +47,7 @@
   shell: |
     set -o pipefail
     if [[ $(df -T / | tail -n 1 | awk '{print $2}') == "btrfs" ]]; then
-        btrfs fi usage --kbytes / | awk '/^.*Free / {print $3}'| sed 's/\..*//'
+        btrfs fi du -s --raw / 2>/dev/null | awk '/[0-9]/ {print $2*1024}'
     else
         df -BK / | awk '!/^Filesystem/ {print $4}' | sed 's/K//'
     fi