Make nova security groups more pluggable

This patch moves the nova security group code out of nova/compute/api.py
into nova/network/security_group. It also removes any query to the database
from security group api into the nova security group driver. This allows
security group drivers the ability to decouple themselves from storing
security group information in the nova_db.

Change-Id: Ib183515a0418203c8bcc88176e3a1498d7333300
Merge "Make nova security groups more pluggable"