Special casing for Ironic VIRT_DRIVER in exercises

Ironic on Devstack does not currently suppport security groups or
anything volume related.  Add some special casing so exercises can
be run without error against the Ironic driver.

Change-Id: I2a54d8cca06fc17894e74b4401af9423cef95635
diff --git a/exercises/volumes.sh b/exercises/volumes.sh
index 0d556df..1dff6a4 100755
--- a/exercises/volumes.sh
+++ b/exercises/volumes.sh
@@ -41,6 +41,9 @@
 # exercise is skipped.
 is_service_enabled cinder || exit 55
 
+# Ironic does not currently support volume attachment.
+[ "$VIRT_DRIVER" == "ironic" ] && exit 55
+
 # Instance type to create
 DEFAULT_INSTANCE_TYPE=${DEFAULT_INSTANCE_TYPE:-m1.tiny}