Add security_group_rule to objects registry

This adds the security_group_rule module to the objects registry,
which allows a service to make sure that all of its objects are
registered before any could be received over RPC.

We don't really have a test for any of these because of the nature
of how they're imported. Refactoring this later could provide some
incremental steps to making this more testable.

Change-Id: Ie96021f3cdeac6addab21c42a14cd8f136eb0b27
Closes-Bug: #1264816
Merge "Add security_group_rule to objects registry"