Rewrote cluster.delete()
Rewrote cluster.delete()

Now, delete instances immediately. Due to some difference between
openstack and AWS EC2 response, I used security groups to identify
an instance state and delete keypairs if they are only associated
with terminated instances(in case of AWS) or with no one(openstack).

Signed-off-by: Shashank Sahni <48273395e0436587f8f1beebda52d256c947abb2@gmail.com>

siel-iiith/MultiStack,siel-iiith/MultiStack