Make security_group_get() more flexible about joins

This makes security_group_get() more flexible about the column
names it takes as join targets. This will let us pre-load things
on the instances that get fetched out.

Related to blueprint compute-manager-objects

Change-Id: Iadbcb7d469eb55add5ada33580c896c7186c98f0
Merge "Make security_group_get() more flexible about joins"