Migrate from using glance image-list to openstackclient

Modify instance of `glance image-list` to `openstack image list`,
in an effort to migrate all commands to unified CLI.

Change-Id: Ieef661d209606e088fbf6a3a394f9a6577667618
diff --git a/exercises/volumes.sh b/exercises/volumes.sh
index 1dff6a4..504fba1 100755
--- a/exercises/volumes.sh
+++ b/exercises/volumes.sh
@@ -68,10 +68,10 @@
 # ------
 
 # List the images available
-glance image-list
+openstack image list
 
 # Grab the id of the image to launch
-IMAGE=$(glance image-list | egrep " $DEFAULT_IMAGE_NAME " | get_field 1)
+IMAGE=$(openstack image list | egrep " $DEFAULT_IMAGE_NAME " | get_field 1)
 die_if_not_set $LINENO IMAGE "Failure getting image $DEFAULT_IMAGE_NAME"
 
 # Security Groups