Fix a lazy-load exception in security_group_update()

When security_group_api is nova, the update_security_group api raises http
500 error: group rules fails with lazy load exception. Because the rule
infos of the security group will be return in response body, but the rule
attribute hasn't been preloaded.

Closes-Bug: #1244018
Change-Id: Ie07879ac22e4add90f75968778bf334915e8349c
Merge "Fix a lazy-load exception in security_group_update()"