Make fake_instance handle security groups

This makes the fake_db_instance() helper create model-like security
group objects in the returned instance if a list of names is
specified. This is required to test anything that uses security
groups on the instance.

Related to blueprint compute-api-objects

Change-Id: I4a7d91c939af8c0dc4e0b3e29c477a859d85fd3e
Merge "Make fake_instance handle security groups"