Fix Instance objects with empty security groups

This makes sure that a DB instance with an empty list of security
groups does not result in an object with an unset attribute, thus
triggering a lazy load on access (and ultimately an exception).

Related to blueprint compute-api-objects

Change-Id: Ib33b05853e9d067cbac5d3e87824a68e9935b822
Merge "Fix Instance objects with empty security groups"