Delete security group that contains instances should not be allowed

Refer: http://docs.amazonwebservices.com/AWSEC2/latest/CommandLineReference/index.html?ApiReference-cmd-DeleteSecurityGroup.html

Rohit, I started putting the code in to check for instances upon deletion , but it's unclear to me - how do you remove instances from the security group prior to deleting the security group?

John,
   This is the closest I could find: http://docs.amazonwebservices.com/AWSEC2/latest/CommandLineReference/ApiReference-cmd-ModifyInstanceAttribute.html. But notice that you change the security group but cannot remove a security group from an instance. The only thing you can do is terminate all the instances and then delete the security group. I think we should still put in the check for instances upon deletion and raise an exception.

Since this hasn't seen activity for a while, I'm going to take a stab at it.

Fix proposed to branch: master
Review: https://review.openstack.org/4154

Reviewed: https://review.openstack.org/4154
Committed: http://github.com/openstack/nova/commit/3dc539bcb0d9031f81076ac2e1870918400150ed
Submitter: Jenkins
Branch: master

commit 3dc539bcb0d9031f81076ac2e1870918400150ed
Author: Russell Bryant <email address hidden>
Date: Fri Feb 10 19:01:10 2012 -0500

    Don't allow EC2 removal of security group in use.

    Fix bug 817872.

    This patch modifies the behavior of removing security groups via the EC2
    API to better match the EC2 API spec. The EC2 documentation says that a
    group that is still in use can not be removed.

    A new function has been added to the db API to find out whether a
    particular security group is still in use. "In use" is defined as
    applied to an active instance, or applied to another group that has not
    been deleted.

    Unit tests have been updated to ensure that an error is raised when
    these conditions are hit.

    Change-Id: I5b3fdf1da213b04084fe266c1a6ed92e01cf1e19