Last modified: 2014-09-23 04:50:45 UTC
When adding a new security group, we might be interested in applying it to some existing instances. The "manage instance" list does not let us add (nor remove) any security group. The workaround is to add the rules to an already applied security group or to the default one.
It's currently not possible in Nova. This is a holdover from EC2, and a quite annoying one. There's a blueprint for supporting this in a future version of the Nova API, though.