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/boot_from_volume.sh b/exercises/boot_from_volume.sh
index dff8e7a..d756685 100755
--- a/exercises/boot_from_volume.sh
+++ b/exercises/boot_from_volume.sh
@@ -44,6 +44,9 @@
 # the exercise is skipped
 is_service_enabled cinder || exit 55
 
+# Ironic does not support boot from volume.
+[ "$VIRT_DRIVER" == "ironic" ] && exit 55
+
 # Instance type to create
 DEFAULT_INSTANCE_TYPE=${DEFAULT_INSTANCE_TYPE:-m1.tiny}