The "default" OpenStack Networking security group is created automatically when creating a tenant network. The Compute service previously always checked for the presence of this "default" security group when launching instances. As a result launching a virtual machine instance when no tenant network had been created ended in error:
SecurityGroupNotFound: Security group default not found.
The Compute service has been updated to only check for the existence of the "default" security group when a tenant network for the instance to use exists.