Add SecurityGroupRule object

This adds the beginnings of the SecurityGroupRule object, with
linkage back to parent and grantee groups. It is not currently
complete enough to serve as a full implementation, but provides
what we need for virt drivers.

Related to blueprint compute-manager-objects

Change-Id: I4099007c8d4e1c8deaa74cd001cb477a9f164261
Merge "Add SecurityGroupRule object"