Skip Nova exercises if Nova is not enabled

This allows for ./exercises.sh to complete sucessfully when nova is not
enabled / installed.

Change-Id: If969e14f5106c15007146e8fad1da27d131828c8
diff --git a/exercises/aggregates.sh b/exercises/aggregates.sh
index 1b1ac06..d223301 100755
--- a/exercises/aggregates.sh
+++ b/exercises/aggregates.sh
@@ -43,6 +43,10 @@
 # Test as the admin user
 . $TOP_DIR/openrc admin admin
 
+# If nova api is not enabled we exit with exitcode 55 so that
+# the exercise is skipped
+is_service_enabled n-api || exit 55
+
 # Cells does not support aggregates.
 is_service_enabled n-cell && exit 55