2.5x increase in default volume backing file size

The current default volume backing file size of ~2Gb seem unfeasibly
small and precludes running exercises that require multiple volumes
and/or snapshots to be created.

We increase this to ~5Gb.

Change-Id: Id6b8d122be919d1b0041288e81b735af21958ff7
diff --git a/stack.sh b/stack.sh
index 2c30bf8..dafe664 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1638,11 +1638,10 @@
     # volume group, create your own volume group called ``stack-volumes`` before
     # invoking ``stack.sh``.
     #
-    # By default, the backing file is 2G in size, and is stored in ``/opt/stack/data``.
+    # By default, the backing file is 5G in size, and is stored in ``/opt/stack/data``.
 
     if ! sudo vgs $VOLUME_GROUP; then
         VOLUME_BACKING_FILE=${VOLUME_BACKING_FILE:-$DATA_DIR/${VOLUME_GROUP}-backing-file}
-        VOLUME_BACKING_FILE_SIZE=${VOLUME_BACKING_FILE_SIZE:-2052M}
         # Only create if the file doesn't already exists
         [[ -f $VOLUME_BACKING_FILE ]] || truncate -s $VOLUME_BACKING_FILE_SIZE $VOLUME_BACKING_FILE
         DEV=`sudo losetup -f --show $VOLUME_BACKING_FILE`