security groups deletion improved

Due to some unexplainable dependency error while deleting idle security
groups, the current ec2.delete_security_groups() implementation tries the
deletion in loop and keeps on removing the deleted security group from
the list.

Signed-off-by: Shashank Sahni <shredder12@gmail.com>
Releasing Elastic IPs at the beginning of delete

Moved snippet of releasing elastic IPs to the beginning of
cluster.delete()

Signed-off-by: Shashank Sahni <shredder12@gmail.com>