Add InstanceList.get_by_security_group()

This provides a way to get a list of instances that are in a given
security group. This alternative to SecurityGroup.instances is
much simpler and avoids the potential for passing around a
SecurityGroup object with a ton of Instance objects attached.

Related to blueprint compute-manager-objects

Change-Id: Iea282ea3559da00a03121d6eff59125a5fbae0b2
Merge "Add InstanceList.get_by_security_group()"