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/client-env.sh b/exercises/client-env.sh
index cc518d9..4a0609a 100755
--- a/exercises/client-env.sh
+++ b/exercises/client-env.sh
@@ -132,7 +132,7 @@
         STATUS_GLANCE="Skipped"
     else
         echo -e "\nTest Glance"
-        if glance image-list; then
+        if openstack image list; then
             STATUS_GLANCE="Succeeded"
         else
             STATUS_GLANCE="Failed"