Orphaned group members

[kaufers]

When destroying a group the first operation that is done is to free it (so that the nodes can be drained, etc):

https://github.com/docker/infrakit/blob/master/pkg/plugin/group/group.go#L162

If there is a problem deleting the instances (for example if the instance plugin has an error) then the group is no longer being managed (so we cannot destroy it again) but the instances still exist.

This is related to https://github.com/docker/infrakit/issues/628; it seems like we need to continue to attempt to remove the instances associated with a group that does not exist.