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/neutron-adv-test.sh b/exercises/neutron-adv-test.sh
index 6679670..5b3281b 100755
--- a/exercises/neutron-adv-test.sh
+++ b/exercises/neutron-adv-test.sh
@@ -134,7 +134,7 @@
 }
 
 function get_image_id {
-    local IMAGE_ID=$(glance image-list | egrep " $DEFAULT_IMAGE_NAME " | get_field 1)
+    local IMAGE_ID=$(openstack image list | egrep " $DEFAULT_IMAGE_NAME " | get_field 1)
     die_if_not_set $LINENO IMAGE_ID "Failure retrieving IMAGE_ID"
     echo "$IMAGE_ID"
 }